Teru Mikami screwed up by going for his notebook when Kiyomi Takada was kidnapped by Mello, which lead Near to the notebook ... If it hadn't been for this one mistake, which allowed Near to successfully replace the notebook with a fake, Light would have won. Unusually for a Spanner, this wasn't a result of stupidity but of Mikami thinking too much like Light: he did exactly what Light would've done in his situation, with the limited information available. In the manga, Light even acknowledges this.
She gummed up the works all the time, but without Misa, there'd be no Rem, and then who knows how long Light would be desperately scrambling for a way to push L out of the way to his New World.
Misa is also a spanner for L, since up until the point that Rem and her were introduced, there was no way for Light to kill L, since he couldn't find out L's real name without the shinigami eyes.
A fairly stupid shinigami, is partly responsible for Mello's escape from the police raid, because he sat still and did exactly what he'd been told to do, rather than taking his Death Note back the minute its current owner died, which would have meant that Mello couldn't have his "give me the notebook" scene.
That wasn't the worst thing he did. The worst thing he did was wrecking Light and Misa's alibi by revealing the fake Death Note rules. Had that not happened, even if Near suspected Light he would have no base for his suspicion, and Light would not have handed over Misa's Death Note to Mikami. And in true Didn't See That Coming fashion, Light didn't even learn about Shidou's existence until the damage was already done. Any other instance on this page either would not have happened or could not have damaged Light's plans if Shidou hadn't done that. It wasn't as though that would have saved Light. After all L was already going to test and see if the fake rules were legit and Near would've done the same. The only thing that occurred because of him is that they didn't have to perform the test.
While we're at it, we can't forget the damage to Light's plans done by his own dad Soichirou, whose attempt to bring Mello in the old fashioned way rather than kill him right then and there led to Mello's survival of the police raid, which eventually led to the incident with Mikami described above.
Mello in the anime certainly counts as a non-stupid example. The purpose of adding him to the show was to add someone straightforward and somewhat reckless to wreck the other main characters' quiet, roundabout and complicated plans. There is a simple, unexpected genius in just driving up in a motorcycle, tossing smoke grenades into the crowd and grabbing your target instead of laying an elaborate trap, now isn't there?
Better is a bit of opinion felt on Near's part. L already gave them the structure to finish the case. After all they didn't have to start from scratch like he did. Plus, L nearly had the case solved on his own. It took a manipulated shinigami to take him down. In fact, it wouldn't have taken Near and Mello to solve the case if someone had the guts to A: Check and see if the fake rules were legit, and B: Keep an eye on Light and Misa. The only edge they had was the fact they were 2 people rather than 1.
Mugen from Samurai Champloo is a prime example. In the first episode, Mugen dispatches a horde of mooks and then approaches their boss, who then explains to Mugen how he is the son of the town's corrupt governor. Any attempts to harm the son of an important official will be a death sentence. He then confidently asks Mugen if he got all that. Mugen responds with, "Not a word," and then attacks him.
Mugen also manages to thwart a hostage situation simply because he was only interested in getting his kicks fighting and couldn't care less about the hostages.
Similarly, Mihoshi from Tenchi Muyo! actually has a distinguished service record with the Galaxy Police, but the backstory indicates it's largely a result of her bumbling in and causing too much chaos for any dastardly plan to hold up.
In the original OVA it was suggested she actually had had a mental breakdown from being overworked which turned her from a top cop into a comic ditz — as hinted by Kagato's comment about her past exploits. Later versions, though, lack this detail and instead are all just lucky enough to be teamed up with Kiyone, who usually can get the job done in spite of Mihoshi. Yet as evidenced by her enormous reports, Mihoshi still seems ridiculously thorough.
In Tenchi in Tokyo, we have three of these: Washu, because the villains always tend to underestimate her, and in this case this lets her decode Yugi's plans; Sasami, because Yugi got distracted by her desire to keep her out of her schemes since she's her Morality Pet (and this is pointed out by Matori, twice); and specially Sakuya, who despite being Yugi's "shadow", in the end turns out to be more independent than she should've been…
In the first Tenchi movie, Tenchi Muyo In Love, Washu's plan to capture KAIN and prevent him from killing Achika gets derailed when KAIN decides if he goes, he's taking Achika with him. It gets worse when Noboyuki is dragged along, too.
In Mazinger Z, thanks to BrockenBall, Koji is able to rescue the kidnapped civilians. Not to mention the fact that in the same series, he saved Koji's life several times, gaining Koji's respect in the process. And in the Mazinger-Z versus Great General of Darkness, as a squad of Mykene Warrior Monsters are trashing Mazinger-Z he suddenly interfered. He only deterred them for several seconds, but those seconds were all that Tetsuya needed to intervene with Great Mazinger and save Kouji's life. And because Kouji did not die, he could help to save the world in Great Mazinger and UFO Robo Grendizer.
In Great Mazinger, if he would have not flirted with Jun and followed her when she was buying clothes, the good guys would be dead by the THIRD chapter.
In the Mazinkaiser movie, he pretty much saved the world (which at that point is a Crapsack World). The Mycene's plan to send one of their general to attack Koji while he is out of Mazinkaiser is completely foiled because Boss attacked the general with Boss Borot.
In Neon Genesis Evangelion, everything is going perfectly for Gendo, until Rei FINALLY gets some self-awareness and realises that no matter how fucked up Shinji is, he would still be a better choice for godhood than a) the lunatic standing with his hand in her chest (literally) b) the group of lunatics who are killing everyone Rei has ever known. So the world is destroyed, recreated and in the end everyone who want to live is allowed to live. Or SOMETHING...
In Super Dimension Fortress Macross, the Zentradi plans to capture the SDF-1 Macross would have succeeded without a hitch were it not for the unpredictable fighting methods of the defenders, as well as the Glamorous Wartime Singer Lynn Minmay who is insanely good at rising the heroes's morale.
In Higurashi: When They Cry,the only arcs in which the village is safe are Watanagashi and Meakashi because Shion kills Rika in secret, and so the mass gassing can't take place within the time that it would take for the virus to incubate, and by the time they did figure out that Rika was dead, oops, it doesn't look like the villagers are going to kill each other after all. This doesn't really make things better for the main characters, though. After all, most of them are still dead. It does, however, clue a mysterious third party in on what's going on; the one aware of the "Groundhog Day" Loop.
The cat in Code Geass nearly succeeds at exposing Lelouch and wrecking all his plans simply by accidentally getting its head stuck in his Zero mask and wandering off with it. Before long, the entire boarding school has caught wind that Lelouch is chasing a cat across campus and the situation balloons into a race against time as Milly Ashford, the mischievous president of the student council, offers a bounty to anyone who can catch the cat and obtain the "embarrassing secret" she assumes it's carrying.
Ironically, Lelouch could count as his own Spanner: on multiple occasions, his attachment to his friends (especially Suzaku or Shirley, but especially his sister Nunnally) have caused him to make moves that jeopardize his plans or even the entire rebellion he leads. The first occurrence in R1 is where he lets Villetta live, because he thought she was no danger to him. When was that? Second episode. The big ones are where he ignores his own unstable Geass, leading to the death of Euphemia, and where he abandoned his followers for his kidnapped sister shortly afterward, his trait continued on in R2. This came to the logical conclusion when his Roaring Rampage of Revenge for the murder of Shirley, carried out against a cult whose members included scientists and children, which ended up being a large part of Prince Schneizel's efforts to turn the Black Knights against him - the centerpiece being the fact that their beloved leader possesses a Mind Control eye and quite possibly forced them all into obedience (he didn't).
Suzaku Kururugi is both this and an Unwitting Pawn. When Marianne brought him to the Sword of Akasha (possibly thinking he'd side with her out of love for her stepdaughter Euphemia, or because of his friendship with her "vessel" Anya Alstreim)... he sided with Lelouch instead.
Not only that, but he messes up Schneizel's plans, as well. Schneizel was planning to eventually kill off his father and become Emperor, but Suzaku realised this and volunteered to help... right in front of several Genre Savvy subordinates who were very worried about doing something like that, Cornelia (who had never considered such a thing, but went along with the idea anyway)... and the big one, Gino Weinberg aka Knight of Three, who's highly loyal to the Emperor. Who escaped with his mecha and told Bismarck Waldstein, the Emperor's personal sword. This made Schneizel's attempt to blame it on Lelouch or the Black Knights completely impossible.
Villetta is really one of the biggest ones for Lelouch, as she figures out his identity as Zero early on (not to mention her part in Shirley's emotional descent, becomes part of the Britannian security detail spying over Lelouch due to her knowledge of and arguably immunity to Geass, and is later a key part of the betrayal in R2 19, as she is the convincing factor for Ohgi, and subsequently the rest of the Black Knights, even with a cursory amount of evidence of Geass (with respect to the truth). The latter of which ends up pushing Lelouch towards the Zero Requiem.
And why does she become such a problem? Because Lelouch is too much of a gentlemen to kill a woman who is unarmed and, by that moment, no threat to him. Bad decision, there.
One of the biggest would have to be Euphemia. When Brittanian occupiers were murdering Japanese civilians by the dozens, it was easy for Lelouch's violent revolution to gain followers. But then good natured Euphemia is put in a position of power; she starts treating the Japanese people with respect, tries to find a compromise solution that both Brittania and Japan can live with, and just like that all of Lelouch's popular support goes bye-bye. Lelouch actually admits defeat to Euphemia, calling her his most formidable opponent.
Mireille Bouquet of Noir may be the least deadly of the assassin girls in the series (which only means that she has to obey the laws of physics), but her ability to tell the Ancient Conspiracy that it can go screw itself and her surprising faith in her partner brings down a major portion of it.
Midori Sugiura from Mai-HiME threw a big wrench in the Big Bad's plans to take over/destroy the world through the "winner" of the battle royale by enlisting the help of the most unlikely of characters: Miyu the Robot Girl, whom she rebooted.
The Movie of Yes! Precure 5 has the movie's Big Bad, Shadow, steal the device the heroines were using to collect the MacGuffinMons of the series, then uses his mirrors to attract every last one of them into the device. With the girls watching, he ends up using it to wish for world domination. It seems that he's won, but... nothing happened. Why? Urara completely forgot to add the last of the MacGuffins to the device, meaning that it's not complete and therefore doesn't work. Oops! The scary part is that the villain, by all accounts, would have won if Ms. Cure Spanner hadn't messed everything up.
Happens again in New Stage 3 - Nozomi's the one who frees the other Cures from their dream worlds, thanks to the fact that she doesn't like being given things on silver platters.
In Chrono Crusade Aion admits that his plan failed because he didn't take Rosette Christopher into account. "No one could predict the actions of such a foolhardy young woman!" Of course, in the anime adaptation the main characters are closer to being Unwitting Pawns...
Monster. The perfect suicide that Johan planned as the end of his Gambit Roulette is ruined when an alcoholic townsperson angrily shoots him in the side of the head before Dr. Tenma can kill him.
Heroic Gambit Example: Between them, Heiji Hattori and Ai Haibara screw up a plan Detective Conan had worked out to get information on the Men in Black. Haibara, despite being an otherwise intelligent and calculating person, felt enough of an obligation to Conan that it made her insistent on going to Conan. She showed up just as the plan was going his way, and when her presence opened the door for everything to get shot to hell. Heiji's role in the Spannering was in deliberately leaving Haibara the means to locate Conan in the first place in case she chose to do exactly what she did.
Another, more indirect, example has an assassination attempt by the Men in Black foiled because of a little boy who kept "Ding-Dong-Ditching" one of the operatives' homes. This act ultimately put Conan in a position to foil the plot.
The fifteenth movie has Touma Tachihara, a boy who wakes up from his years-long Convenient Coma at the worst moment possible for the Big Bad because he's the only person able to identify him... since he witnessed the murder he committed in the past. Pretty much the whole movie's plot relays on whether Touma's Trauma-Induced Amnesia will be undone or not.
Hot-BloodedBadass Normal Sanosuke Sagara from Rurouni Kenshin once threw a wrench in Magnificent Bastard Shishio's plans by sneaking close to his battleship and wrecking it with some bombs given by his friend Katsu, an explosives expert. This prevented Shishio from bombing Kyoto. And Shishio openly acknowledged Sanosuke's role in this.
Rune Soldier: The idiotic sorcerer Louie ruins the Secret Weapon when he takes a leak in an unusual spot. After that he punches his way through a impenetrable barrier ruining Plan B as well.
In Basilisk, Tenzen's perfect plan to become the last man standing is ruined when Princess Oboro, whom he broke mercilessly throughout the series, regains her sight and uses her power to prevent his last revival and make his head explode.. This after Oboro has already put a serious kink in his plans by sealing her own eyes in the first place, to avert using her powers against her own clansmen..
Ashita no Nadja has a huge Spanner situation. When their Smug Snake boss Hermann betrays them, Rosso and Bianco ruin his plans as they let a kidnapped Nadja escape and tell her where to find their reports, which is ultimately Hermann's perdition.
Also, Hermann thought that hiring Rosemary as Nadja's Body Double would give him an advantage. WRONG! Rosemary, despite being just 13 years old, proved to be much smarter and more independent than Herman believed, flat-out telling him that if he tried to discard her, she'd reveal their plans and he'd go down with her. Suck on that, Smug Snake.
In Naruto, Big Bad Tobi outright calls Naruto one of these after he defeated Pain and convinced him and Konan to turn against Akatsuki, which led to Tobi losing Pain's resurrection technique, Amegakure's support, and the technique needed to seal the two remaining tailed beasts into the Gedo Mazo.
Killer Bee also threw a spanner in Akatsuki's plans, when he faked his own capture to go on a vacation.
Killer Bee does it again, when he foils Kisame's attempt to capture him, helped with Kisame's sword betraying him and Killer Bee's brother turning up to save him. However, it later turns out that Kisame throwing away the fight was all part of the plan.
Sasuke is one to Itachi's plan, which was ironically meant to keep Sasuke safe. Sasuke's actions lead to the exact opposite of what Itachi had planned for him, culminating in him becoming a criminal wanted by two villages.
Tobi becomes a spanner to Itachi's plans, as after Itachi's death, he starts revealing Itachi's secrets to people that he wanted to hide them from.
Itachi has foiled or complicated the plans of nearly every single person or organization he's come across in the series. In chronological order, the Uchiha clan's plan to revolt against Konoha, Danzo's plan of taking over Konoha, Orochimaru's plan of stealing Itachi's sharigan eyes and later preventing him from taking over Sasuke's body, Team 7's attempts to rehabilitate Sasuke (in order to preemptively foil any plans Danzo might have after Hiruzen's death), Akatsuki's plans as The Mole, a failed attempt to foil Tobi's plan to brainwash Sasuke. Even the resurrected Itachi foils Kabuto's plans by sealing the resurrected Nagato, and then traps Kabuto himself in an infinite loop. Phew! He also foils his own plans for Sasuke multiple times by needlessly messing up with an already vengeful Sasuke's mind.
The real Madara escapes from Edo Tensei's release becoming immortal with infinite chakra and stamina, having already blown away much of the Shinobi Alliance without breaking a sweat before the aforesaid escape.
Tobi puts a spanner in the real Madara's plan as he decides to execute the plan all by himself.
Three of the 12 Sisters in Coyote Ragtime Show, Oct, Nove, and Diesse, accidentally sent an enemy ship flying into a bomb capable of an Earth-Shattering Kaboom... and nothing happens, revealing the bomb above the planet Graceland to be a fake. Spannered further by the militants who act on this information, who are unaware that the fake bomb was a distraction to keep people from finding the real bomb, which is already on the planet and was moments away from being disarmed before they killed the people negotiating for it.
Katekyo Hitman Reborn! has a mid-level mechanic incidentally named Spanner that performs a Heel-Face Turn along with Irie in an effort to take down the Future Arc's Big Bad Byakuran using his technical knowledge of the base and Millefiore. It would have slightly more effective if Byakuran wasn't an ultimate Chessmaster and the in-series King of Xanatos Speed Chess forcing the epic battles, struggles and hardships just for his own amusement.
Luffy from One Piece honestly doesn't care about the World Government's rule, or for that matter any of the Big Bads he faces. He just won't let anyone he cares about get hurt and will go to any lengths to protect his True Companions. Hence his defeat of two out of Seven Warlords of the Sea, the destruction of Enies Lobby, and the assault upon the most powerful jerkasses in the world. Soon afterwards we add the first successful mass-jail break from Impel Down and his assault upon the Marine HQ, just to save his big brother from being executed. The surrounding politics and larger conflict don't enter his mind at all.
As a matter of fact, Luffy and his crew have a tendency to do this wherever they go. Captain Kuro, Crocodile, Eneru, CP9, and Shiki (from Strong World) all had very elaborate plans that were a stone's throw away from fruition, but then along came Luffy and crew. All of the above made the mistake of underestimating them, and paid dearly for it.
They also (unsurprisingly) stumbled upon the villainous plot for Fishman Island on their way to the New World, and thwarted that one as well. It helped that Big Bad Hordy Jones, in spite of knowing everything that the Straw Hats had done before their hiatus, dismissed the nine of them as no threat and used Evil Gloating to outright provoke them. With exactly one ally joining the Straw Hats to fight, Hordy and his army of 100,000 found themselves on the receiving end of what may be the most devastating Curb-Stomp Battle in the series as a result.
Luffy's proficiencies to this trope have been lampshaded during the Impel Down arc, when the jailers have to deal with two simultaneous riots on top of Blackbeard's invasion, both as a result of Luffy sneaking in.
Magellan: Nothing is going our way! How could one intruder spark so much mayhem?!
In the Assault of Marineford Arc, Little Oars, Jr. caused the Marines strategy for fighting Whitebeard to backfire when his body blocked a piece of a huge siege wall they were trying to activate from rising.
Buggy also qualifies in that same arc. His desire to grandstand and show off to a world audience led him to capture a video den-den mushi, meaning that the World Government didn't have total control over what information was getting out of Marineford when they started slaughtering the pirates mercilessly with the Pacifistas, or when Blackbeard betrayed the World Government, indelibly humiliating them in the process.
Sabo. Not only did he catch Doflamingo off guard, he caught the audience off guard. His appearance in the colosseum allows him and Luffy to switch places (after a heartwarming reunion), so Luffy can save Law without worrying about the Mera Mera no Mi falling into Burgess's hands. The funniest part about this is that his appearance can be largely blamed on Doflamingo's actions — chances are Sabo wouldn't have had any reason to go to Dressrosa, had it not been for the Mera Mera no Mi. So, Doflamingo's attempts to take advantage of Luffy's Spanner in the Works tendencies to screw up Law's plan ended up creating another one to mess with his plan.
What makes it even better is the fact that Doflamingo is just that Dangerously Genre Savvy, knowing that even with all of his resources and strength, underestimating the Straw Hats would be a fatal mistake…and despite his best efforts, he still makes that mistake. He sends some of his best fighters to deal with them: one to attack the weaker members still on the ship and prevent them from leaving, and one to take advantage of Sanji's nature as a Chivalrous Pervert to find out everything about their plans. The first one fails because the one he sends is tricked into giving one of the Straw Hats just enough leeway to attack her and knock her out, neutralizing her powers. The second one fails because Sanji manages to win the heart of the one he sends, which results in her revealing his plans.
A brilliant example is shown with Usopp. Unable to move or fight back, he still manages to knock Sugar, a Devil Fruit user that was single-handedly responsible for maintaining a Masquerade that had kept Doflamingo in power for the last ten years, unconscious, neutralizing her power. All hell breaks loose, and Doflamingo is MAD.
Dennou Coil protagonist Yasako is revealed to be this in the final episode. When Yasako was about seven years old, she stumbled into a hidden Space being used to help a girl her age, later known as Isako, cope with the death of her older brother. Yasako encounters a simulation of this brother and her affection towards him sparks Isako's insecurities and fears of losing him, resulting in the creation of Miss Michiko, which is the cause for many problems throughout the series.
Akiyama sees Nao Kanzaki as this in regards to the Liar Game. The Liar Game's producers break even by collecting on the players' debts, and profits when the winning contestants forfeit half of their winnings to Opt Out of the game. Nao, however, has consistently been using her winnings to pay off other players' debts while increasing her own as a result. Akiyama believes that Nao has the potential to completely bankrupt the Liar Game by doing this since by the end, her personal debt will have gotten so high actually collecting would be impossible.
Senoo Kaori in Saki does this to Mako in the same way as the poker and billiards examples listed in Real Life. Mako's playstyle is based entirely on having watched experienced players all her life. Kaori is a complete beginner. The result is that because Kaori has no idea what she's actually doing, Mako found it impossible to read her discards and ended up losing to her.
The military high command in Strike Witches were running a conspiracy to abolish the Strike Witches in favor of their own methods. Their plan was all in place and ready to go until Yoshika attempted peaceful contact with a Neuroi. Since the high command's methods involved illegal use of Neuroi technology, Yoshika's actions forced them to act prematurely and made the other Witches realize they were hiding something.
Full Metal Panic!: Though not a ditz or stupid like some of the other examples, kidnapping Ordinary High-School Student Kaname means that you've instantly lost, because she tends to figure out ways to screw up your best laid plans. Not to mention the whole "bodyguard with Humongous Mecha" thing she has going, and how said bodyguard is the most hardcore Badass Normal the world has ever seen and such.
In Fullmetal Alchemist, it turns out Scar's older brother was the first to figure out Father's Ancient Conspiracy and think up a countermeasure before the series even starts that allows the heroes to fight back.
You can't help but pity Father. Despite being an Eldritch Abomination who spent centuries erecting a massive plan to gain godhood, fate sent an entire legion of fools to ruin his day. In addition to Scar's brother, you've got Ling Yao and his entourage, who can sense Homunculi and just want the secret of immortality; May Chang, who uses alkahestry instead of alchemy and is on the same quest as her half-brother Ling; and even Scar himself, whose thirst for revenge almost derails the plan several times.
Hohenheim is the ultimate spanner. Father originally let him live and gave him half of the philosopher's stone made from Xerxes because Father came into existence when the slaveowner who owned Hohenheim, who had no name at the time, used Hohenheim's blood to create Father, and this origin allowed Father to gain a physical body identical to Hohenheim's as well. So Hohenheim, after becoming an expert on Alchemy and living for centuries in the hopes of one day derailing Father's plans, finds love in a normal woman named Trisha. They have two boys together: Edward and Alphonse. These two children, along with their father, are the main reasons Father can be stopped.
The Second Greed to Father, who certainly wasn't expecting him to regain his old memories and betray him a second time. Not only does Greed betray Father but he also joins the heroes to fight against him. Greed managed to weaken Wrath (greatly helping Scar) and helped the Briggs soldiers hold the Central fort. He was also crucial in the final battle as he managed to absorb some of Father's souls and distracted him long enough for the heroes to make him use up his Philosopher's Stone shield. Most importantly, after being absorbed by Father (who at this point, still couldn't die due to his regeneration abilities), Greed greatlydamages him by using his carbonization powers to turn Father's shell into charcoal, thereby rendering Father unable to regenerate. All that was needed left to destroy Father afterwards was a single punch from Edward.
An earlier example would be at the end of the first Dragon Ball hunt. Pilaf has summoned the Dragon and is about to wish for world domination, but he never expects one of the good guys to escape from their prison and change the wish...like Oolong did.
Pilaf: I wish to rule-
Oolong: The panties off a hot babe!
And afterwards, preparing to kill the heroes for their interference, Pilaf's palace is destroyed by Goku, having turned into a giant monkey monster.
And it's not the only one. Natsumi Murakami is the owner of an artifact which hasn't been found/used/invoked in centuries, obtained from her Pactio with Koutarou. Said artifact is basically an Invisibility Cloak... and she uses it to bring a whole rescue team to the place where a captive and unconsciousAsuna is about to be used by Fate to rewrite the whole Magical World. Fate's horrified face when he sees what happened is priceless.
In Durarara!!, one of the main reasons Izaya hates Shizuo so much is because his emotional volatility, Too Dumb to Fool nature, and firmly sustained belief that Izaya is responsible for 99.9% of everything that goes wrong in Ikebukuro makes him liable to become one at any moment. Shizuo ends up doing exactly this in volume 6, when Izaya's attempt to frame Shizuo and get him out of the way for the moment doesn't go quite as planned and instead brings even worse people down on Izaya's head.
In Baccano!'s Flying Pussyfoot arc, it seems like everyone is this for everyone else, in a plot that was already capable of being summed up in one word: "clusterfuck." Long story short: the Lemures plan to hold the train passengers hostage in return for Huey Laforet's release from prison. Ladd Russo and his buddies plan to kill a bunch of people For the Evulz. Czeslaw Meyer plans to smuggle explosives into New York and sell them. Jacuzzi Splot and his gang of delinquents plan to steal Czeslaw's explosives and sell them. Isaac and Miria have already pulled off a robbery and plan to use the train to make their getaway into New York City. Daily Days agent Rachel just wants to hitch a ride. And Rail Tracerdoes not approve of all these shenanigans going down on his train. All of these plans disrupt one another in spectacular fashion and end in varying levels of success based - with one or two notable exceptions - on how sympathetic the characters involved are.
Pokémon: Ash often manages to throw spanners into his rival's tactics due to his tendency to make seemingly dumb choices of Pokemon (bad type match-ups, significantly smaller Pokemon, etc.) and then do the unexpected with them.
Also the Team Rocket trio. Jessie, James and Meowth almost always manage to create problems for Cassidy and Butch, other Rocket agents, or other villain teams by trying to steal their prize. As several characters have noted, they are actually much better at fighting other villains than fighting the heroes.
In Pokémon Special, Giovanni has a Game Breaker Deoxys that can Forme Change instantaneously at will. Yes, he's pretty much unstoppable...until Bill and his buddies figure out how it works and disable it.
During the finale of Pokémon: Zoroark: Master of Illusions, Kodai wins, he gets the Time Ripple. All plant-life in Crown City begins to die! But suddenly it turns out to be all an illusion by Zoroark. But her illusions shouldn't work on him, he's got a device for that! Well remember that cute little Zorua he's been kicking around the whole movie? The one who tried his hardest to stop him but failed? Well turns out a lucky little bite in their last scuffle, unknown to ether of them, broke Kodai's illusion canceler, bringing Kodai's plan crashing down.
In A Certain Magical Index, Touma often derails the complex plans of any character by simply showing up and kicking the crap out of every bad guy in sight. Subverted in that Laura Stewart and Aleister Crowley are Magnificent Bastards and can factor him into their schemes. Later, Aleister has a Villainous Breakdown when Shiage defeats Meltdowner, raging that it's impossible and ordering his death in an effort to stop him from derailing his calculations any further.
In the Makai Arc of YuYu Hakusho, Yusuke's decision to hold a tournament was a last ditch effort to eliminate Yomi and Mukuro, since Yusuke knew he had no hope of taking either of them in a fight. However, due to Kurama and co.'s lucky presence, and Mukuro's bugging of Yomi's estate, Yusuke's idea prevents all hell from breaking loose: if Yomi were to simply attempt to kill Yusuke, Kurama, as well as his own elite fighters, would side with Yusuke. While Yomi would be capable of killing them all, he'd expend at least a third of his power doing so, leaving him vulnerable to Mukuro and Hiei, all of this forcing Yomi to accept Yusuke's proposal.
In Beast King Golion, Honerva's plan to take down Altea by killing Shirogane falls victim to two spanners- one was Kurogane in Red Lion coming up with the idea to push the Deathblack Beastman Galcia into some magma, and when Galra attacks again before Altea could train a replacement, Fala takes Blue Lion.
In Tiger & Bunny, Maverick's longtime manipulation of both HeroTV and Barnaby for his own ends is utterly destroyed by a cascading series of spanners in the works, beginning with Kotetsu, who completely inadvertently foils Maverick's attempt at wiping potentially incriminating knowledge from his memory and forces him into a much grander plan to try and remove him from the picture...
...which falls apart thanks to Kotetsu's former boss, Ben Jackson. Maverick wipes the memories of Wild Tiger's secret identity from the HeroTV crew, but overlooks Ben, leaving him able to come to Kotetsu's rescue when the other heroes are out to arrest him.
And, to a lesser extent than the above, Judge Yuri Petrov of the Justice Bureau, aka Lunatic. Maverick could not possibly have anticipated that Yuri had done extensive research into Wild Tiger and Wild Tiger's civilian identity after crossing his path as Lunatic, or that Lunatic's obsessive sense of justice would lead him to come to Kotetsu's aid just in the nick of time to keep Kotetsu from being arrested by the fake Wild Tiger.
And finally, Kotetsu's daughter Kaede, who goes into Sternbild to help prove her father's innocence. When she approaches Maverick to ask for help, he has no idea who she is or that she's recently manifested the power to duplicate the powers of any NEXT she touches, and simply pats her on the head... unwittingly giving her the ability to undo his brainwashing of the other heroes with a well-timed Emotion Bomb. Even once he's realized who she is and taken her hostage, he and Rotwang still fail to properly account for her power, and thanks to coming into contact with Blue Rose shortly before their capture she's able to prevent Rotwang from executing the heroes in his deathtrap.
Ringo Oginome pretty much tells Shoma Takakura that he became one for her "Project M" in episode 11 of Mawaru-Penguindrum. This, because Shoma is the first person to question said "Project M", in which Ringo wants to reach almost all the possible extremes to bear the child of Tabuki-sensei since it means she'll live the life of her dead older sister Momoka; among other things, Shouma tells Ringo that she's herself and not Momoka, which later makes her think twice in regards to having sex with a Love Potioned!Tabuki.
There's another one whose spanning "powers" act on a bigger scale: Masako Natsume. In her backstory episode, we find out that she doesn't want to be used by Sanetoshi at all and only goes along with his plans resentfully to keep her little brother Mario alive. Later on, she refuses to burn her half of the Destiny Diary, which serves as some sort of seal on Sanetoshi's power; the entire diary does get burnt to bits by episode 23, but Masako's refusal holds him off long enough for Ringo to learn the Fate Transfer spell.
Nine times out of ten, bad guys in Gundam tend to get their plans uprooted because of this.
The entirety of the Principality of Zeon is just one big spanner against themselves in Mobile Suit Gundam's One Year War. Among them being:
M'Quve refusing to give Ramba Ral the new Doms, leading to his death and later that of the Black Tri-Stars, depriving Zeon of experienced and talented soldiers.
Sayla and Amuro spotting a Federation traitor just before Operation Odessa, managing to foil his plans.
Ghinias Saharin snapping and attacking, leading to the death of his own men that his sister Aina had brokered for a cease-fire to get them out for medical attention.
Zeon soldiers mistakenly thinking the Gundam had been destroyed early on in the Battle of Solomon, leading a talented and seasoned pilot like Anaval Gato to break away and miss fighting Amuro.
Zeon blowing a lot of their funding on expensive Mobile Armors or experimental, extremely limited-focus Mobile Suits;
Ghiren firing a superweapon too early just to kill his own father, also wiping out LOTS of Zeon's troops. Then a pissed off Kycillia executes Ghiren for Patricide, leading to the powerful Delaz Fleet (that Gato is a part of) to bail. And to think, they had spent 9 months at that point in a stalemate and were really close to winning...
And, of course, the ultimate one: three Zeon soldiers go to Side 7 to investigate goings on in there and discover the Federation building Mobile Suits there. Had they not attacked the factory...
In Mobile Suit Gundam SEED Destiny, Durandal ends up throwing a spanner towards himself and his Destiny Plan by trying to assassinate Lacus Clyne. Had he not decided that Lacus had to die, then Lacus's boyfriend Kira and the Archangel crew wouldn't have gotten back together to stop the new war that just started, and may had even accepted the Destiny Plan.
In Mobile Suit Gundam ZZ, Haman Karn and her Neo Zeon group had successfully obtained Side 3 through her massive Xanatos Gambit. It seemed that the Earth would be next... had Glemy Toto not decide to reveal himself as a clone of Ghiren Zabi and initiate a civil war with her.
In Mobile Suit Gundam Wing, Romefeller's War for Fun and Profit scheme fell apart because of Princess Relena Peacecraft. Romefeller's leader, Duke Dermail, invaded Relena's homeland because she was espousing Total Pacifism, which started to undercut Romefeller's authority. Dermail offered to call off the invasion if Relena dissolved the nation and agreed to become the figurehead Queen of the nations unified under Romefeller's banner, and she accepted. He didn't expect Relenato reject that whole "figurehead" thing and continue advocating peace, leading to others within Romefeller to admit they're very tired of war.
In Gundam Build Fighters, Chairman Mashita of PPSE creates arcade game-like machines that animate Gundam model kits, which have become a worldwide phenomenon and made him a nice chunk of change. And then along comes Reiji, the prince of the otherworldly kingdom from which Mashita stole the unusual particles which make Gunpla Battle machines work. Afraid that Reiji's victory would give him the spotlight and the opportunity to expose Mashita's theft, the Chairman begins rigging the World Tournament in the hopes of eliminating Reiji...but due to his piloting skills, the model-building talents of his partner Sei Iori, and the True Companions bonds they share with each other and most of the tournament's other participants, every attempt thusfar has failed.
In Honoo No Alpen Rose, both Jeudi and her mother Helene were this. The first, via appearing in the Durant mansion and thus throwing a wrench in the local Smug Snake Toulonchamp's plans, which involved him having his own daughter Mathilda as his spy as well as Jeudi's Body Double to twist the Durants's arms into helping him; it goes double when she starts singing the Alpine Rose song when she's being taken away, letting Helene know the truth and identify her as her daughter instead of Mathilda. The second, via crawling towards Toulonchamp in the middle of a Hostage Situation and holding on his leg while he's about to kidnap Jeudi at knife point, distracting him enough to let Jeudi's boyfriend Lundi pounce on him and setting Jeudi free.
However, Friederich Brandel aka Jeudi's dad/Helene's husband was the biggest one. The aforementioned Alpine Rose song? It was his most famous theme, and not only it doubled as a symbol of La Résistance against Those Wacky Nazis but as his biggest token of love for Helene after they became Star-Crossed Lovers. Jeudi singing it to prove her identity to Helene totally undid all of Toulonchamp's plans in regards to the Durants, and before that it was one of Jeudi's few memories about her own past as well.
Also, Toulonchamp's spy Anna commits the VERY dumb mistake of trying to kill Jeudi via dropping a pot on her head, which the girl barely manages to escape from. This not only scares the crap out of Jeudi's grandparents (reducing their willingness to collaborate with Toulonchamp no matter what he says), but it also terrifies Mathilda herself and leads her to, despite not defecting to his side, drop some of the info she has to Lundi. This ultimately forces him to directly go to the Durants's home, right in the moment when General Guisan (a highranked military leader as well as a friend of the Durant family who knows that Jeudi is the real Durant girl) shows up as well...
In Princess TutuAhiru is this to Drosselmeyer. Made more ironic by the fact he was the one to bring her into the story to get it moving. It did; it the direction opposite to what he wanted.
In Anatolia Story, Queen Nakia needs a special girl's blood to perform a powerful Blood Magic that will kill her stepson Kail and give her the chance to make her own son Juda the heir to the Hitite Empire. Too bad that the girl she brought into Hattusa as her prospect Human Sacrifice, Yuri Suzuki, turned out to a Plucky Girl who decided to fight back to protect herself, the became the concubine of the same prince Nakia wanted to murder, and she not only ended up becoming Nakia's strongest rival but also one of the most influential people in the nation.
Eren being a titan shifter in Attack on Titan derailed the other titan shifters's plans by causing them to shift their focus towards Eren.
In Saki, Nodoka doesn't believe in superstition, and therefore doesn't notice when something weird is up. In her match against Eisui's Hatsumi, she derails the plans of Teams Himematsu and Miyamori players, who did notice Hatsumi's ability and were trying to stop it from triggering, by just playing normally.
A fairly common stock superhero plot consists of bad guys managing to finally defeat or capture the heroes, only to have the entire plan foiled by the appearance of an unexpected new recruit.
As an homage to the above-mentioned Justice League story, Grant Morrison's JLA run had a two-part story where Connor Hawke (the second Green Arrow) is forced to rescue the team after they are ambushed and incapacitated by the Key.
In another Justice League story penned by Morrison, Prometheus's debut involved him taking down each member of the League with remarkable ease, having planned out their encounter to the letter. He hadn't planned that Catwoman would've snuck on board the Watchtower disguised as one of the reporters there for the day, and while he's trying to blackmail Superman into committing suicide on national television, she foils his entire plan by giving him a bullwhip to the nads.
In The Transformers IDW series, Shockwave, planning to seed dozens of worlds with super energon so Cybertron wouldn't be mined hollow and die, is attacked by the Dynobots looking for revenge.
It seems that for super-logical Shockwave Dinobots are his natural enemy - the first time when they were created (for the sole purpose of stopping Shockwave from getting to the crashed Ark) he completely dominated them in battle, thrown them down the cliff and started shooting at them from higher ground knowing that they can't even fight back but he overlook the possibility that someone may be stupid enough to attack the cliff itself bringing down the enemy standing on top of it but being buried underneath even deeper and faster than he was.
In a strange way this is the hat of Fantastic Four villain the Mad Thinker - he creates amazingly complex, unassailable plans at the drop of a hat, accounting for every possible action of his opponents. But, every time, there is some random variable (what he calls the "x-factor") that doesn't take into account and buggers up his calculations. Marvel AdventuresSpider-Man once established that the Thinker hates Spider-Man because his precognitive Spider-Sense makes him the one person on Earth who can effortlessly derail the Thinker's schemes without even deliberately trying.
Zayne Carrick in Knights of the Old Republic basically sets in motion the Self-Fulfilling Prophecy of the Jedi — the one that they were trying to escape by killing their Padawans — when he (unintentionally) misses the knighting ceremony and then escapes. He continues to thwart their plans, intentionally or not, ever since.
He also thwarts Arkoh Adasca's plan (with the help of Lucien and Alek) by tricking the Mandalorian leader into thinking Adasca is working with the Republic to trap him, which results in a three way brawl which culminates in Adasca's ship getting eaten by giant space slugs.
Runaways often sees villains' perfect plans ripped to shreds by Badass Normal Chase Stein, who claims to be street smart but as a fellow teammate inquired, "What street? Sesame?"
In The DCU, the Challengers of the Unknown are a team of adventure-seekers who miraculously survived a terrible plane crash, and therefore decided that they would willingly face any danger because, as they always put it, they were living on "borrowed time." It has recently been revealed that this is literally true: because they did not die on their appointed death date, the Challengers are the only people in the world whose fates are not recorded in the Book of Destiny. They can freely disrupt predestined events that would otherwise be literally inevitable, making them the ultimate example of this trope.
Subverted in the Dan Dare series Reign of the Robots, wherein the only person that the Mekon paralyses after capturing the group is Dare's bumbling aide Digby, on the grounds that "he has no brain, therefore there is no predicting his actions".
Groo The Wanderer: One of his more memorable derailings involved him going up against a mind-reading sorcerer. Groo fights his way to the sorcerer's throneroom, is confronted confidently by him... followed by a full page of the sorcerer making strange faces at Groo while the latter stared at him in befuddlement until he finally screamed "There's no mind to read!" and ran away.
In Cerebus the Aardvark, the title character possesses a "magnifier" quality that influences everything and everyone around him to varying degrees. This causes the plans of everyone who tries to do anything that directly involves Cerebus to succeed wildly then crash spectacularly.
Deadpool is frequently seen as such a rogue element that the guy who can copy someone's fighting style completely (Taskmaster) was still surprised by him.
In Cable & Deadpool #50 (the final issue of the series) the Savage Land mutate Brainchild is countering the moves of every other hero in their attack on his base, but none of his predictions of where Deadpool is are accurate; turns out DP took the "Super Mario strategy" and went through the sewer pipes.
Cyclops in the Ultimate X-Men arc Return of the King. Let us recap the situation so far. Magneto has regained his memories and is going on a rampage across the world with his acolytes while he waits for Forge to get his Doomsday Weapon ready. All the X-Men are either captured, killed, or on the run and still have no idea where Magneto's base is. It looks like all hope is lost, when Cyclops, who everyone thought was killed by Wolverine a few issues ago, is taken into Magneto's base as an injured mutant in need of healing. After recovering Cyke busts his way out and clues the other X-Men in on where the base is. All of this leads to a truly epic smackdown against Magneto and saving the world.
In one week-long FoxTrot series, Jason finds it impossible to beat one guardian monster in a video game, as it instantly squashes his character every time he tries. Paige, who almost never plays video games, takes the controller and gets by the guardian by simply walking around him.
Snively describes Sonic as this in the Archie Comics' 200th issue after the blue blur defeats Eggman yet again, causing the doctor to go mad.
Snively: Eccentrics aside, he really is a genius. He can build the most amazing things and plot a hundred steps ahead ... And then there's you. All the building, calculating, and planning in the world couldn't beat you.
Snively's not the only one who's noticed this. Mammoth Mogul has told Sonic that he will not attempt to conquer the world while Sonic's alive. Eggman reasoned himself to a degree of sanity after he realized this, thinking that Sonic's near constant exposure to chaos energy has turned into a literal force of chaos.
Sadly, in Silver's timeline, this turned Mogul into his own spanner - because he refused to act, Princess Sally was roboticized, only being remembered as an unidentified traitor, turning her against the Freedom Fighters and helping to ravage Mobius.
Cebolinha/Jimmy Five from Brazilian comic Monica's Gang is known for "infallible plans" against Monica. They usually work up until a certain point, when "accomplice" Cascăo/Smudge screws up, usually by revealing it was a plan.
In Tintin and the Picaros, Colonel Sponsz plots to have his old enemies (i.e. Tintin, Haddock and Calculus) be the subjects of a fake kidnapping by Tapioca's nemesis (and Tintin's old friend) Alcazar so they can all fall victim to an ambush on a back road. The spanner is a monkey in the road, which causes Alcazar's getaway truck to suddenly swerve the moment it comes into firing range.
In the Fables spinoff The Nearly Great Escape, Jack figures out that Goldilocks is working for Revise because she wears the same style of glasses that several of his minions wear. This is a complete coincidence, but it turns out that he was right anyway.
An early Spider-Man issue has Mysterio convincing Spider-Man that he's going insane, and then posing as a therapist who offers to help our web-headed hero. Of course, Mysterio then sets up more illusions to make Spidey think he's hallucinating again. Spidey's about to have a complete nervous breakdown when J. Jonah Jameson and Flash Thompson, who had both heard about the therapy session and wanted to support Mysterio and Spider-Man (respectively) wander into the house. When they start seeing and reacting to the same "hallucinations", Spider-Man realizes that he's being conned, and swiftly defeats Mysterio. It's later lampshaded when Jameson realizes that Mysterio was on the verge of making Spider-Man reveal his Secret Identity, and that his own arrival torpedoed the whole plan. After that, Jameson is the one who seems like he needs therapy.
Batman, or rather Bruce Wayne, was this to the Court of Owls. For centuries they have used entertainment venues like Haley's Circus to recruit youths and brainwash them into their loyal elite Talons. They had planned to do the same to Dick Grayson (a descendant of one of their most successful Talons), but the deaths of Dick's parents and his immediate adoption by Bruce Wayne afterwards placed him out of their reach.
Margaret from Yoko Tsuno's story The pray and the shadow. Forced by her boss to be his adoptive daughter Lady Cecilia's Body Double and be a part of his cruel plan to get said daughter killed and inherit her wealth, Margaret is shit scared of continuing in the plot, and she secretly contacts Yoko to both help save Cecilia and free herself from her evil boss...
The Minute Men in 100 Bullets become this in the end. The whole series is a giant game of chess, but nobody ever thought to tell the most powerful pieces on the board. They never get a single answer to any of their questions and end up ruining every single character's master plan simply because they have nothing better to do.
A tragic one in Batman: No Man's Land. With Gotham's No Man Land status lifted, The Joker decides to squash that happy feeling by kidnapping and planning to murder every baby born during NML. In the search for the clown and the children, Sarah Essen-Gordon's radio is smashed, leading to Commissioner Gordon to tell her to head back to GCPD HQ to get a new one. What no one realized until too late was that that's where the Joker was hiding! By the time they get there, the Joker surrenders... only after shooting Sarah in the head in front of the kids.
In the Vampirella story "The Resurrection of Papa Voudou" Paul Giraud manages to stagger into the ritual chamber, where he falls beside the altar. As a result, the spell intended to resurrect Papa Voudou splits its power between repairing the villain's mind and restoring Paul's body. Thus, Papa Voudou is raised as a sapient zombie but his body remains decayed.
The Deadpool arc of Ultimate Spider-Man has the titular mouthy Merc being hired to kidnap the X-Men, fly them to Genoshia, and hunt them on TV. Things hit a bump when Spidey winds up brought along for the ride due to poor timing, which throws Genoshia's whole strategy out of whack. Deadpool himself sums up partway in that the hunt isn't going well because Spidey is the only thing they planned for.
Professor X: I'd like to thank you, Mister Parker. Things might have turned out very different if you hadn't been there to balance the odds in our favor.
Fate/stay night's fanfic Chaos Theory opens up with this. It introduces the plots and manipulations of all the antagonists of FSN, and then the narrator calmly declares:
"But mostly? The assorted plans at play here would be going very, very wrong due to the actions of a no-name, no-count, utterly talentless Magus by the name of Shirou Emiya. He had no magic worth mentioning, no combat experience of note, and no plan for or knowledge of the War he was about to enter. He did, however, have one trait that had derailed a countless number of such grand, far-reaching schemes throughout history.
In Ultimate Sleepwalker: The New Dreams, the Green Goblin kidnapped and planned to publicly murder Gwen Stacy as a way to get revenge on her father, Captain George Stacy of the NYPD. While Spider-Man intervened and tried to rescue Gwen, the Goblin had anticipated that and set things up so he'd be able to kill both Spider-Man and Gwen at the same time. Unfortunately, what he didn't take into account was Sleepwalker following Spider-Man to the top of the bridge a Cascnd distracting him long enough for Spider-Man to rescue Gwen and get her to safety. The Goblin proceeded to have a Villainous Breakdown.
The Pony POV Series has one that's notable for the exceptionally long time required for the spanner to actually get into the villain's works. Back in the G3 Universe, which was facing The End of the World as We Know It (actually a Shoot the Dog to avert a Class Z Apocalypse) at the time, Pinkie Pie's best friend Minty just bled to death after their fight with Luna, leaving behind her "spirit" (or at least a piece of herself). Strife, Discord's sister and Spirit of Natural Selection, is fighting the survivors of the doomed world (in order to give them at least the chance to fight for their survival, something she believes is the right of all living things) and sent Heartless-like spirits of "erased" ponies after them. One of these falls into the canyon where Pinkie and Minty's fight took place, wounded in battle. Pinkie, in a split second choice, fuses the piece of Minty with the shadow, which turns out to be that of the G1 Twilight. The result? Twilight Sparkle!
During the Dark World Arc, Discord's little sister Rancor is this. She arrives to get close to Discord and ultimately steal back Destruction's power from him so it can be used for its intended purpose (Destruction's original job as an Anthropomorphic Personification), mortally wounding him in the process. This ends up throwing a gigantic wrench in Nightmare Eclipse/Paradox's plans, allowing Discord to Out Gambit her and alert Twilight to the true natureof the Dark World, forcing Paradox to expose herself to the heroes. The name Rancor chooses after succeeding (Disruption) couldn't be more appropriate. It's this trope especially because, while its implied she knew Paradox was there, she didn't care one way or the other what happened so long as her own mission was a success.
In Evangelion fan-fic RE-TAKE, SEELE finds all their plans foiled by something they could never have anticipated Shinji be aided by Ghost-Asuka and "God" which allows him to destroy the Mass Production Evas during the fan-fic's take the events of "End".
In the Mario Fan-Fic Clash of the Elements, Fawful, Alpha and Mario are this to Cackletta's plans. And Bowser too, for freeing the Star Spirits from their imprisonment.
Nanoha's presence in Game Theory derails Precia's carefully laid plans. But this turns out to be a subversion, because the events that disrupted Precia's original strategy actually made it possible for her to come up with a better plan that works flawlessly.
In Kage, Yua predicts Jade's presence in Meridian will cause Nerissa's plans to go Off the Rails, and any attempts by the elderly sorceress to force Jade to fit in her plans will only cause the schemes to unravel further. Jade is also this to the second season timeline in general, with the changes already starting to add up.
An Alternate Keitaro Urashima repeatedly derails his grandmother's attempts to manipulate him simply by letting others know how manipulative she is. As a result, when she shows up and starts spinning her webs, she winds up proving him right.
Mokoto also runs afoul of this when she threatens Keitaro one time too many. When she draws her katana, she's unaware that some police officers are passing behind her and witness it.
Death Note Equestria: Mer ends up going off Twilight's script in order to ruin her in revenge for Rarity's death as a part of one of Twilight's earlier gambits. Subverted, however, as Twilight had actually anticipated this and factored it into her plans, resulting in Twilight's triumph and Mer's death.
The ending of Layer Cake has the protagonist outclassed not by dumb luck, but by being shot. Because while he was really successful in tricking clever criminals in his Batman Gambit, he ends up shot (and possibly killed, it's a little vague) by a guy whose girlfriend he stole and whom he considered of little importance.
Carlitos Way ends in a similar fashion. After outsmarting all his enemies by the skin of his teeth Carlito ends up getting killed by some random lowlife he mistreated earlier in the film.
Before both above films, you have New Jack City, where drug kingpin, Nino Brown, is able to fast talk his way out of serious prison time, only to get killed by an old war veteran he didn't take seriously during the film.
The Expanded Universe has the entire Imperial Fleet artificially boosted by the Emperor's force powers. The Emperor's death ended up causing the imperial officers to lose control of the situation, preventing them from shooting down the Millennium Falcon before it could destroy the Death Star 2.
There's also Jar Jar Binks, whose clumsiness is more than a match for several tanks.
Anakin Skywalker's destroying the Trade Federation's droid control station in the first prequel was a massive stroke of luck. To the extent that not even he realized what was happening. He just hid in an unmanned Naboo fighter and stuff happened.
The scheming husband in Dial M for Murder is undone because he underestimates the intelligence of Swann/Lesgate, the thug he hired to kill his wife. Swann puts the key right back after using it, rather than keeping it, as his employer expected.
In the recent adaptation of The Pink Panther, it seems like Inspector Clouseau, a seemingly Inspector Oblivious is one of these until the very end, where he reveals that he was a Chessmaster after all. According to Peter Sellers, the original Clouseau qualified as well, but he knew he was a buffoon deep down. Strikes Again had killers from all over the world come after him. He bends over to tie his shoes at the exact right moment... Likewise in the film's Dénouement, Clouseau is unwittingly catapulted onto Dreyfus' Death Ray, destroying it and killing Dreyfus in the process.
Sweeney Todd: The Demon Barber of Fleet Street: Sweeney Todd would have killed Judge Turpin and ended the movie right there and then in the middle had Anthony, who had recently talked to Sweeney about his plan to elope with Johanna in order to get her away from Turpin, not busted into his shop with the judge right there in the room in order to inform Sweeney that he has found Johanna and that she has agreed to the plan. Needless to say, this ends up blowing both the aforementioned plan and Sweeney's attempt to kill Turpin straight to hell.
All throughout Pirates of the Caribbean: Curse of the Black Pearl, Jack repeatedly plots for the most favorable outcome (for himself), but stubborn fool Will Turner and arrogant jerkass Captain Barbossa assume they know best how to get things done, and nearly screw themselves out of their goals frequently. If Barbossa had wanted to cut Elizabeth's throat instead of her hand, Will would've been too late to save her on his own, and if Will had died like he should've when Barbossa ordered the Interceptor scuttled with Will trapped below, Barbossa would never have gotten Will's blood to pay Bill Turner's debt. Near the end of the film before the climactic battle, Jack has everybody where he wants them, but because Barbossa and Norrington don't trust him at all, his plans almost fall apart.
That's what Jack WANTS you to think.
In the 1932 sci-fi mystery film Doctor X the Mad ScientistSerial Killer manages to not only trick the other characters into believing he is innocent but also manipulates them into physically restraining themselves so he can slaughter them at his leisure. Unfortunately he forgot about the Plucky Comic ReliefIntrepid Reporter, who manages to dispatch him in a terrified and bumbling fashion at the last minute.
In The Caper film The Killing, a band of criminals pull off an elaborate robbery of a racetrack. Even though the most of the criminals kill each other off fighting amongst themselves, the Anti-Hero and his Love Interest manage to escape to the airport and prepare to board a plane out of the country with all the loot. However, all their plans are foiled when a dog runs out in front of the luggage train, causing it to crash and spill the loot all over the runway for all to see.
By refusing to sacrifice each other, the passengers on the two boats in The Dark Knight end up being this for The Joker, in that his social experiment in proving that anyone could be corrupted ends in failure.
In Batman Begins, Batman himself is the Spanner towards Ra's al Ghul and the Scarecrow's plan to destroy Gotham.
Oh, the joys of Chicken Run - this is one of many visual gags, and surprisingly one that is relevant to the plot.
In Cast a Deadly Spell, the Evil Necromancer's plan to summon Cthulhu is thwarted when it turns out his daughter was no longer a virgin due to the idiot cute cop nobody had been paying much attention to.
The terrorists from Vantage Point might have gotten away with it, had it not been for a little girl.
In The A-Team, Face's plan would have gone off smoothly if not for Pike having a SMAW.
Pike: Here's what I think of your best laid plans!
* Pike fires the SMAW into the ship's hull*
Cowboy CopJohn McClane'sreal job. When terrorists are confidently moving chess pieces behind the scenes, he knows that all he has to do is look for something sensitive and start whaling on it.
The Adjustment Bureau tries to contain this kind of incidents. But they are not above random chance and unexpected behavior.
Gina in Unknown. Despite just being his cab driver, she saves Dr. Harris' life at the beginning of the movie and twice afterwards, killing Mooks and the Big Bad in the process, which also allows Harris to stop the plan of the Big Bad.
In A Few Good Men, it becomes apparent that the Department of the Navy very much wants the case of United States v. Dawson & Downey to be quietly resolved by a Plea Bargain so as to prevent incident from causing too much embarrassment to the Marine Corps. This plan is undone by the dual spanners of Galloway and Dawson; Galloway, not because she is dumb, but be because she is clever enough to sense something in not right and thus goads Kaffee in doing his due diligence for once instead of rushing to a Plea Bargain, Dawson, for being too hardcore of a jarhead to willingly accept a dishonorable discharge because it would make thing easier for him, and telling Kaffee to take his plea bargain and shove it.
In The Atomic Brain, as shown on the TV series Mystery Science Theater 3000, in spades. The main plot for the villain Miss Marsh was for her to have her brain swapped with the prettiest girl of three chosen. That gets ruined when the homeliest of the three, who had her brain swapped with the cat's, gouges her eye out. Miss Marsh's companion tries to double-cross her, killing her and letting the remaining girl take her money, but Marsh kills him. The scientist, Dr. Otto Frank, pulls one by placing Miss Marsh's brain with the cat's, revealing that he wanted to keep her locked away in the cat so he could use her money to continue his research on reviving the dead. Miss March responds by locking him in the revival chamber and setting the dial to "Frag the entire house".
Had Thor not been there to keep the Hulk at bay the Helicarrier would have crashed and Loki's plan would have more than likely been successful.
The arc reactor in Tony Stark's chest. It prevents Loki from mind-controlling him and turning him against his allies, which would have derailed everything just as much as the Hulk's potential rampage.
The title character of Wreck-It Ralph. His desire to gain the fame and respect his rival, Fix-It Felix, Jr., had, even unwittingly, lead to Sugar Rush's King Candy being exposed as the thought-dead Turbo and the restoration of glitch character Vanellope Von Schweetz.
Roz: Two and a half years of undercover work were almost wasted when you intercepted that child, Mr. Sullivan. Of course, without your help, I never would have known that this went all the way up to Waternoose.
Kirk in Star Trek Into Darkness. By threatening Harrison and offering him a chance to surrender, rather than killing him from afar as ordered, he single-handedly and accidentally derails all of Admiral Marcus' plans. And Spock, who drove him toward that decision.
In The Lady Vanishes an oblivious Iris manages to completely destroy a Nazi conspiracy just because she won't give up insisting that the eponymous lady exists.
Olaf in Frozen was this. With Kristoff having left the castle when Anna was about to die, Hans' plan may have succeeded and given him the throne. But there was no way to account for Olaf, who saves Anna.
Elsa was also this. Her denial of Anna's marriage to Hans and her departure from Arendelle wound up saving Anna as it delayed his plans and gave Anna a lesson in trusting people so quickly while at the same time showing her The Power of Love.
S.W.A.T.: Alex Montel kills his uncle Martin Gascoigne for sticking his hand in The Syndicate's till, then takes Gascoigne's car to the airport to catch his flight home. On the way, he's pulled over by a motorcycle policeman because he has a tail light out. The officer then discovers there's an arrest warrant linked to the license plate and detains Montel "until we can verify who you are." Lampshaded by an FBI agent after Montel's identity is discovered:
"We've been lookin' for this guy a long time. Busted tail light brings him down? That's amazing."
In Ocean's 11, all it takes is Anthony Burgdorf's widow deciding to have his funeral in Las Vegas for Operation Pine Box to go to hell.
The Black Tattoo is an incredibly epic example. Jack Farrel began as an Audience Surrogate who was only there because his best friend Charlie refused to join the secret society dedicated to protecting the world from the Scourge without him, and really had no Lehrer purpose in the main plot. Charlie's foolishness and arrogance get Jack accidentally sent to Hell, but Jack's determination and desire to help his friends eventually drive him to help Action Girl Esme stop Charlie and the Scourge, and it's his actions that ultimately wind up saving the world.
A ditzy cultist hands the newborn Antichrist off to the wrong unsuspecting parents in the beginning of Good Omens, thus setting off a plan that derails Armageddon itself. Of course, this may have all been a bigger Gambit Roulette planned out by Powers That Be.
In the first book of The Hunger Games, Katniss and Peeta are this because the Head Gamemaker panicked when they attempted to commit suicide and decided to make them both victors.
Used and inverted in J. R. R. Tolkien's The Lord of the Rings. In order to destroy the One Ring and defeat Sauron once and for all, Gandalf develops an Indy Ploy / Batman Gambit to sneak the ring right under Sauron's eye into Mordor and Mount Doom, the one place it can actually be destroyed. It almost worked except the One Ring itself spans the plan by finally corrupting Frodo. Then Gollum anti-spans the One Ring's spanning by grabbing it and accidentally falling into Mount Doom. Supposedly, Gandalf knew that the Ring would ultimately corrupt Frodo, especially since it would be the most powerful within the Cracks of Doom, and that another Spanner like Gollum would have to occur for the plan to work.
Merry, Pippin, and Sam are all Spanners to some extent as no one, not even Frodo, wanted them to come along on the quest. Even so, each one of them took actions that enabled Sauron's downfall.
In Isaac Asimov's Forward the Foundation amidst the chaos surrounding high-level plots and counter-plots, Galactic Emperor Cleon I is assassinated by a totally insignificant palace minion, because he (Cleon) was insisting on promoting said peon, against the peon's fervent wishes, from "gardener" to "chief gardener".
Gunner First Class Ferik Jurgen, assistant to Ciaphas Cain, turns out to be the one who most often saves the day, with his combination of being a "blank" who nullifies psychic powers and the fact that he carries a really, really big gun.
Rincewind never wants to get involved in events, being a coward. In Interesting Times, his great ambition is to stay as far away from the villain's Evil Plan as possible. However, he always seems to run away from danger in the direction of even more danger... until he winds up cornered and desperate, at which point he does the right thing in spite of himself.
Some of the more inept members of the Ankh-Morpork City Watch, especially Sergeant Colon and Corporal Nobbs, fit this trope.
The A Song of Ice and Fire book A Feast For Crows has Lysa. And Jaime. And Robb. And Allerie. And Roose. And so on and so forth. Westeros is so awash in spanners it's a wonder that it's ever experienced peace. But of course, as anyone who's finished Dance with Dragons knows, Varys is the motherfucking king of all spanners.
As early as the first book, Bran witnessing Jaime and Cersei's incest and getting pushed out a window is a huge spanner. It was a complete coincidence and no one- not even chessmasters like Littlefinger or Varys- could have predicted it, yet it directly sparked a chain of events that would eventually lead to the War of the Five Kings, the main conflict of the series.
Joffrey can also be a spanner at times: While for the most part he's fairly predictable, his cruelty and insanity occasionally lead him to do things that no one would expect, such as ordering Ned Stark's execution, despite his mother and Varys and Grand Maester Pycelle all telling him to let Stark join the Night's Watch instead. And let's not forget that even before he became king, he was the one who sent an assassin to murder the comatose Bran, just because he'd heard a passing comment by his father about how the boy would be better off dead.
Edmure Tully combines this with Nice Job Breaking It, Hero when, on his own initiative, he leads a successful attack on Lannister forces that not only derails Robb's plans to trap said Lannister force in their territory but delays them enough to receive word of Stannis' attack on King's Landing. This allows Tywin Lannister and his forces to go be Big Damn Heroes, derailing Stannis's bid for power.
One of the lingering plot points is the state of Robb Stark's successor. Robb 's death has lead to a power vacuum in the North. Knowing that this may happen, Robb named his half-brother Jon his successor in the North, ahead of Sansa. Considering Sansa was, at the time, a political prisoner, this will screw over both the Lannister's and Littlefinger's ambitions to take control of North if this comes to light.
Kender, gully dwarves and gnomes in the Dragonlance series...especially (by their very nature) the kender. While all of the above races have the ability to change events in the past through time travel, due to their origins as races created by the Greygem of Gargath (pure Chaos-in-a-rock), kender have innate fearlessness, insatiable curiosity, guileless but mischievous personalities, and chronic kleptomania as racial traits. Tasselhoff Burrfoot, for example, is both the Unwitting Pawn of Raistlin's evil schemes and the only person unpredictable enough to screw them up. One of the most dreaded sounds on Krynn is the sound of a kender saying 'Oops.'
Given the choice between being locked in a room with a hungry dragon or a bored kender, anyone with any sense picks the dragon.
Remember, the cruelest thing one can do to a kender is lock him up. The cruelest thing one can do to anyone else is to lock them up with a kender.
This trope is sort of lampshaded in the second Honor Harrington novel where the protagonist explains to her subordinate that the best swordsman in the world doesn't fear the second-best one, but the worst swordsman in the world, because he can't predict what the dumb son of a bitch will do.
Apparently there is some truth in that. An inexperienced swordsman is more likely to do something that gets both combatants killed than an experienced one trying to avoid dying.
A similar thing happens in the Age Of Unreason series, where a guy is killed by someone who cannot fence at all; he automatically assumed his attack was a mere feint, because no fencer would make such a clumsy attack. Too bad his opponent is not a fencer...
A character in Echos of Honor is known as "Silver Spanner" Maxwell, after a Noodle Incident involving a dropped spanner produced spectacular (and expensive) results, six years previously.
Aivars Terekhov pulls this in The Shadow of Saganami when his ship happens to encounter the same ship, using two different identities on two different planets, running weapons to terrorists on two different planets. Using evidence found on that ship, Terekhov figures out the antagonists' plan and quickly rushes to put a stop to it.
The best part is that they only noticed the discrepancy because Aikawa, the midshipman on watch, was bored, so he ran a detailed analysis on a random freighter.
Arnold Giancola is this on a couple of levels. His actions doctoring diplomatic correspondence led to a resumption of the war when his plan to score a political coup backfired spectacularly. Despite other characters believing he was, he actually was notpart of the Ancient Conspiracy making a power grab but instead acting on his own. Later, his purely accidental death further derails investigation meant to examine his original actions.
Mesa has been the victim of more than a couple spanners since becoming the Big Bad of the series:
Following the Green Pines incident, they placed the blame on Manticoran agent Anton Zilwicki, as he was confirmed to be there and they Never Found the Body. Coupled with Zilwicki and Victor Cachat's transport breaking down and requiring more time to return home than they would've needed otherwise, they had every reason to think he was dead. Not only are Zilwicki and Cachat alive, they also successfully extracted Dr. Herlander Simőes, a defector. So not only do they get caught in a lie, now that Zilwicki can challenge their version of events, but Simőes has just enough information to alert Manticore and Haven of the Mesan Alignment.
In Shadow of Freedom, Mesan operatives have been going to worlds on the Solarian League's borders, posing as Manticorans and offering aid to various La Résistance movements. The idea was to damage Manticoran PR by having these movements' failures appear as though Manticore offered aid then left them in the lurch. This plan is Spannered by one world seeking to contact Manticore independently, alerting them of the plan.
John "Anjin-san" Blackthorne in Shogun is a rather magnificent one, as circumstances force him into a key role in the Gambit Pileup of choosing Japan's next shogun in the year 1600. And he's based on a real guy, to boot.
Tom Clancy's Executive Orders: A pair of domestic terrorists spend most of the book preparing a massive cement truck bomb to kill Jack Ryan, driving it all the way across the country, dodging roadblocks put up as a result of The Virus spread by the otherBig Bad of the book, only to be pulled over and arrested by a random Highway Patrolman just doing his job when they panic.
The titular assassin of Frederick Forsyth's The Day of the Jackal seems well ahead of the international police effort to stop his attempt on Charles de Gaulle until some things come up to derail his plan. Just one of many comes up when his seduction of a baroness to gain a hiding location falls apart when said baroness eavesdrops on a call with his informant, forcing him to kill her and letting the police make him publicly wanted as a common murderer.
And ironically, his last spanner was de Gaulle himself, who leaned forward to kiss a recipient on the cheeks instead of shaking his hand like the Jackal expected, therefore making the Jackal's shot miss and giving Lebel enough time to stop him.
And in fact, main protagonist Sparhawk is essentially a personification of this trope. As the "man without destiny," no one can really divine or guess what exactly he's going to do in the future...not even the gods, who are scared shitless of him.
In James Swallow's Warhammer 40,000Deus Sanguinius, Rafen shocks Arkio's forces by being alive. Inquisitor Stele is quite glad that he will die in single combat, because he had landed in the plans by a fluke and quickly grown to "the most serious nuisance." Of course, he wasn't dead at that point...
Mat Cauthon in The Wheel of Time almost literally personifies this trope. He isn't stupid, but he's rarely clued into just what exactly is going on around him. Despite this he foils many schemes, especially when he's actively trying not to.
And similar to the line mentioned in Honor Harrington above, when the White Tower's weapons master tells Galad, Gawyn, and Mat a story about history's greatest swordsman, who was only defeated once in his entire life - by a random farmer with a stick.
In Rainbow Six, Eddie Price's pipe-smoking after success missions is the first in a series of little things that clue The Dragon in to the true nature of Rainbow despite efforts to hide it. Carlos the Jackal's fellow criminals, in carrying out an attack to try and get him freed, gets the team some good publicity that gets them to view the Sydney Olympics for free and puts them in the right place to foil a vital part of the villain's plan. One of the villains' captives manages to get off an email, the investigation into which eventually helps lead the way to them. One of the named minions tells The Dragon, hitherto ignorant of the truth, about the extent of the plan, prompting a vital Even Evil Has Standards moment that causes him to go to the good guys with the information, allowing the case to be cracked.
In the Dale Brown novel Executive Intent, Wayne Macomber would have avoided capture by GRU agents had he not stumbled over a random civilian in the wrong place at the wrong time, who proceeded to inform the police and make the already suspicious GRU agents take action.
John Lennon's short story "A Spaniard in the Works" has very little to do with this trope. Or, for that matter, anything else.
In Bystander the villains run into a very severe example of this, when they have a great plan to capture Lucretica, and are foiled by two details. One, they have a severely incorrect estimation of her power level, and two: Her feet don't touch the ground.
In Harry Potter and the Half-Blood Prince, Draco Malfoy's act of disarming Dumbledore completely derails the gambits that both Dumbledore and Voldemort had in place in regards to the Elder Wand. Though with a bit of luck it ends up working out great for Harry himself.
Fireheart in Warrior Cats. He completely ruins Tigerclaw's plans by running into the cave where Tigerclaw was during a battle and beating Tigerclaw up.
In The Dresden Files novel Changes, they are able to narrow down the location of where the sacrifice will occur because even though the records for the first shipment were destroyed, the red court still had to transfer another shipment due to the fact that the previous shipment was incomplete due to minion carelessness. As such they needed to keep one copy of the records intact until after the final checkup and they kept the records in the van they needed to use to transport the goods. As such Harry is able to narrow it down and eventually find Chichen Itza, enabling him to blow the Red Court's plan sky high.
Harry in general serves as this for many a grand villainous plan, in much the same fashion as John McClane.
The Gatekeeper himself lampshades this in Cold Days.
The Gatekeeper: Unwittingly or not, virtually your every action in the past few years has resulted in a series of well-placed thumbs in the adversary's eye.
In Queste, the fourth book of Septimus Heap, Jenna and Beetle are this to Tertius Fume's plan to kill Septimus with the Queste.
In The Dance of Time, the final book of Eric Flint's Belisarius Series, there is a side plot of a Malwa assassination team tasked to kill Byzantine Emperor Photius and Empress Tahmina but keeps getting foiled by unexpected changes of plans of their targets. The team follows them for thousands of miles while the plot of the rest of the novel occurs around them. At the end they run across the fleeing Malwa emperor and the Big Bad (currently inhabiting the body of an eight-year old girl). They kill him and his guards and wound her which makes it possible for the good guys to finally achieve complete victory. They get rewarded by being sent into exile with the series' Manipulative Bastard.
In The Demon Headmaster books, Dinah manages to be both this and an Unwitting Pawn at various points. The Headmaster can easily hypnotize her, and she's very close to being his greatest asset, but she's just intelligent enough to shake it off and bugger it all up at the last minute. For the record:
Book One is the one where they meet. He doesn't know her capabilities.
Book Two has him not knowing her new name.
Book Three has him not even realising she could figure it out.
Book Four has him specifically targeting her for her DNA.
Book Five has his clone, who doesn't even know she exists. Dinah stumbles on this one purely by accident.
The final book has him attempting to demoralize her. He almost wins, and would have had Dinah not had a last-second burst in hearing capability.
In one of Francoise Rivier and Michel Laponte's Jonathan Cap books, recurrent character and local Plucky Girl Juliette becomes this. She has her appendix removed in a Parisian private clinic and notices that both her doctor and the nurse in charge of her are acting strange, notifying Jonathan's Kid Sidekicks Alex and Nico about it so they can call Jonathan and investigate. It turns out the doctor is the Big Bad of the book, with a complex plan involving an Arabian prince and his Body Double (the Big Bad's "disciple"), and the nurse is his forced accomplice because he threatened to kill her if she didn't collaborate. The plan would've gone smoothly, had the Big Bad not been pretty much forced by the circumstances to be the doctor in charge of Juliette's emergency surgery...
In the historical novel Wings of Dawn: Thomas, to the Druids. Umar and Hadad, to Sir William. The Mamelukes and the bandit troops, to everybody.
Mark Twain's short story "Luck" speaks of Lieutenant General Lord Arthur Scoresby, V.C., K.C.B., etc., a complete idiot who got on the fast track to war hero-ism because he got his left and right mixed up in battle and accidentally led his regiment right into where the other side was preparing a surprise attack, and drove the ambushers into full retreat because there was no possible way they would be discovered in time, and so didn't prepare.
Miss Emily Dorothea Seeton, created by Heron Carvic. She's difficult to describe. As far as she's concerned, she's a retired teacher the police use as a sketch artist, not an amateur investigator. So when the bad guys come after her, she usually has no idea why they're doing so. Her reactions usually end up with the villains in custody, the case closed, and quite possibly police trying to write reports that make sense.
In The Maze Runner Trilogy, Thomas and Minho finally get in on some Spanner action at the beginning of The Death Cure . Everything previous to that was planned by WICKED and there was no point at which they were not under WICKED's control.
in Academ's Fury, Sarl and the Vord would have won if it wasn't for one person: Canim ambassador Varg. He managed to figure out their plan, and because he doesn't like the duplicity involved, goes out of his way to stop it. They did plan for him, but they figure that because he is hated, nothing he says will be believed. And they would have been right, had Varg not ended up skipping diplomacy and outright dragging Tavi to the Vord nest, alerting Tavi to the threat and allowing him to prepare.
Tavi's aunt Isana was also an unknown factor. She knew about the Vord from Calderon, and it was because she cut a deal with the Aquataines (whom she had hated before) to get support for the legionnaires fighting the Vord in Calderon, and got Invidia Aquataine to counterattack the Vord, breaking their attack.
In Elsabeth Soesten No Good Deed...]] Elsabeth and Hieronymus accidentally foil Cuncz's plans to steal incriminating information about himself from Father Garnerius when Garnerius hires them to recover the reliquary (where the Abbot stashed the documents) stolen by Cuncz's agents. Cuncz just shrugs off the setback and hires them to finish the mission instead.
Live Action TV
In the Doctor Who story City of Death, Duggan, the detective who seems to have gotten into his line of work just because he likes hitting things, derails the villain's multi-millennial scheme with one thoughtless, well-timed punch.
In the episode The Pirate Planet, after all the planning to destroy the Mentiads by both a cyborg pirate captain and a tyrannical Queen Xanxia in disguise, the Mentiads and the Doctor manage to do this trope... literally.
In the story "The Caves of Androzani", unusually, the Doctor's role in the story is limited to frantically attempting to get him and Peri out alive. His mere presence, however, inadvertently causes the entire messed-up Androzani society to implode. The Doctor brings down a corrupt government accidentally.
Get Smart, of course; Maxwell Smart is more likely to defeat KAOS by accident than on purpose.
A British Sketch Comedy program parodied this. A man is buying a camera, and is shown one that is "totally idiot proof". He then smashes it on the table. "What did you do that for?" "Well, I'm an idiot." The shopkeeper then shows him a camera made out of concrete.
In an episode of Wallander, the title character avoids a fatal bullet by tripping over a conveniently-placed rug.
In TNA, Taylor Wilde and Lauren Brooke derailed Dr. Stevie's attempts to turn Abyss into his puppet. Despite using drugs, physical abuse, and mind games to keep him in line, Dr. Stevie didn't count on Abyss falling in love with Lauren. Then, when he ordered Abyss to attack Taylor, he didn't count on her being Lauren's best friend...
Married... with Children: Kelly Bundy tended to mess up whatever plan she became involved in, given her role as The Ditz and Brainless Beauty. It's even lampshaded by Peggy at one point as the Bundys and the D'Arcys are being arrested by the police, when she notes that it probably wasn't a good idea to let Kelly in on the plan.
Angel has this when a sorcerer's plan to sacrifice his virgin daughter for power is thwarted when it turns out she hasn't been a virgin since she was in her teens. She even dated one of his mage bodyguards, who probably knew that she had to be a virgin for the sacrifice to work, and shuffles awkwardly offscreen after she points him out.
The Season 5 finale of Weeds features an unusual instance of a smart character acting as fate's tool: Shane Botwin's murder of Pilar, a brilliant criminal who acted as puppeteer for Estaban, the Mexican stock exchange, and Mexican government as a whole. Essentially, Shane and his croquet mallet accomplished in a mere second what Nancy and Guillermo had failed to do in half a season, and those two burnt down an entire town without getting caught...
SG-1 was once called to help Thor to serve this very purposenote Actually a subversion: the Asgard didn't need stupid people, they needed people who thought very differently from the Asgard, as the Replicators were perfectly capable of countering Asgard attempts, but had no experience with humans.
Jack O'Neill: So what you're basically saying is you need someone dumber than you?
General Hammond: I'm sorry, Thor, but we need SG-1 here.
Sam Carter: I could go, sir.
Jack O'Neill: I dunno, Carter, you may not be dumb enough.
In the third series of Primeval, Helen has a plan to kill the first hominids and thus erase humanity from existence. She very nearly succeeds, if not for one desperately hungry raptor.
The main characters in The Good Guys are generally competent cops (yes, even Dan Stark) who solve major crimes largely by stumbling into them while investigating something much smaller.
Shotaro Hidari, despite being the main character of Kamen Rider Double, gets this from every single one of the show's Chessmasters. Neither Ryubee Sonozaki nor his wife Fumine, AKA Shroud, ever considered that the overly emotional and all-too-human detective could possibly be a factor in their plans, especially since Ryubee's Terror Dopant form emits such raw Primal Fear that Shotaro can barely think in his presence, and Shroud was planning on partnering up Ryu Terui (Accel) with Philip since they're both immune to that effect. Neither did the True Final BossJun Kazu, the Utopia Dopant, thanks to his Reality Warper powers. Shotaro manages to show them all up, overcoming his limitations with a combination of Hot Bloodedness and loyalty to his True Companions that lets him make the impossible possible, even defeating Utopiawithout transforming into Double. Arguably lampshaded by Shotaro being tied heavily to the concept of the Joker — he's both the Wild Card (the element nobody saw coming) and the Trump Card (the key to victory).
In Heroes Vs. Villains, Tyson served as a Spanner In The Works for Rob's gambit by serving as an Unwitting Pawn in Russell's gambit. The Villains tribe was divided between Rob's and Russell's supporters, with Rob leading 6-3. However, Russell had an immunity idol, which meant that when the tribe voted someone out, he (or whoever he gave the idol to) could stand up and play the idol to prevent any votes cast against them from counting. Rob wasn't sure whether the idol would be played by Russell or by Russell's closest ally, Pavarti, but Rob had a plan to guarantee they could get rid of either Russell or Pavarti. He would have three people vote for Russell and three for Pavarti; then, regardless of who played the idol, the other one would have three votes. Even if all three people on Russell's side voted for one person, it would have meant a 3-3 tie between votes to get rid of Russell or Parvati and votes to get rid of whichever Rob supporter Russell targeted, leading to a tie-breaker that Rob's side could easily win. However, the plan fell through because Tyson actually let Russell (his alliance's enemy) tell him who he should vote for. Rob had assigned Tyson to be one of the three people to vote for Russell. However, Russell told Tyson that he would sacrifice Pavarti and use the idol to protect himself, leading Tyson to think that he should vote for Pavarti because votes against Russell wouldn't count after Russell played the idol. Russell had other plans, and gave the idol to Pavarti. After the votes were cast, Pavarti played the idol, meaning that the final vote count was four votes (including Tyson's) against Pavarti that didn't count, two votes against Russell, and three votes cast by Russell and his allies against... Tyson. It was three to two, and Tyson was sent home. Afterward, Tyson admitted to the camera, "I am a victim of my own stupidity." His action was one of five official nominees for the dumbest action in Survivor history.
Sash fell victim to three Spanners during Nicaragua. He was in a very good position in the game, having just ousted Chessmasters Marty and Brenda in quick succession, but then his closest allies Kelly and NaOnka decided to quit at the same time, leaving him without an ideal final three (Kelly did very little in both strategy and physical play, and NaOnka was universally disliked by the other tribemates). He managed to quickly get himself into a secret alliance with Chase, Holly, and Jane, but then Fabio, who was both the alliance's obvious target and had been faking the part of the Dumb Blonde up until that point, planted the idea in Chase's head that Jane needed to be taken out before anyone else. Sash proceeded to make the plan his own, which in turn led to Jane revealing the alliance at the next Tribal. From there, Sash could only helplessly watch as Fabio went on a string of Immunity Challenge runs, keeping him safe to the very end, which forced Sash to turn on his alliance and vote out Holly. The end result? A Final three of Sash, Chase, and Fabio, with Chase getting four votes, Fabio winning with five, and Sash with... none at all.
An episode of Fringe features a villain whose intelligence was boosted so he can perfectly predict anything that will happen and set up ridiculously complicated scenarios to kill people and escape from the agents chasing him. He's foiled by Olivia not actually being from his universe, just brainwashed to think she is. So she doesn't recognize a sign indicating a low oxygen area, and doesn't pause to grab an oxygen mask, allowing her time to dodge the stack of pipes he thought would kill her.
This trope has made the lives of the investigators on CSI: Crime Scene Investigation much, much easier over the years. Whether it's people discovering a dead body before it can be completely dissolved or buried, witnesses who unwittingly provide evidence that ties a murderer to a crime scene, or a Heroic Bystander who catches a Peeping Tom that also turns out to be a serial rapist, various members of the public have helped the CSIs bring a lot of criminals to justice in many different ways.
Tragically, Lancelot became this for Arthur and Guinevere on Merlin despite repeated promises to himself and Merlin that he would never come between them. However, Morgana resurrects him after his Heroic Sacrifice, robs him of his free will, and forces him to seduce Guinevere (who was enchanted to respond to his advances).
In the first volume of Heroes, there was a large and complicated plot that involved blowing up New York in order to usher one of the characters into the White House. The only thing stopping it from being a Gambit Roulette was the fact that the people responsible were basing their convoluted plot on the works of an artist who could paint the future. And it was working, hell, it almost did work. Unfortunately for them, a certain Future Badass with the power to control the Space-Time continuum didn't like the result, and traveled back five years to give a message to one of the present day characters. This guy had no idea that there was any sort of plan, he just thought it all happened naturally, but the message he delivered set off a chain of events that ended up ruining the plan at the last minute.
Leverage: In "The Gold Job" Hardison demands, and is given, the opportunity to run his own con by Nate. Hardison plans an elaborate Batman Gambit based on video game theory and it's working very well, until the people he's manipulating decide that it's not worth the effort to keep jumping through his hoops. In an ending scene Hardison receives a letter Nate wrote earlier in the day which outlines the three things the plan needed to succeed (which Hardison's plan made possible). Nate then explains how he plans his cons to anticipate the possibility of spanners; he starts with the crude, ugly basic plan, and then plans the elaborate, beautiful, intricate plan from there.
Supernatural: Naomi calls Castiel this by name, and says that he has never followed orders.
The Winchester brothers might also qualify, as it is their unwillingness to do as they're told that derails the angels' apocalypse plans.
Salem: John Alden's mere presence in Salem is screwing up the witches' preparations for their Grand Rite.
Invoked, if only as pastiche (which Morrissey adores, as we know) in The Smiths' title track "The Queen is Dead":
So I broke into the Palace With a sponge and a rusty spanner
...referencing a peculiar event—long forgotten by most, perhaps—though the unlikelihood of Michael Fagin's misadventure conveys this trope's inherent flavor of the unexpected. In the BBC's retrospective, we learn "[t]he Queen was only able to raise the alarm when he asked for a cigarette." In more security-conscious days, who'd imagine such a Royal contretemps could occur at all?
A mediocre GM can do this to players. When the players have a good plan which bypasses the intended challenge handily or solves a challenge with minimal fuss, a good GM may let the players have their moment in the sun and save those encounter notes to recycle them with new window dressing. A mediocre GM will have the players' plans fail for increasingly implausible reasons. A poor one will have a deer in headlights look as soon as the players are Off the Rails.
Example: Give the PCs the Eye of Vecna, you get some fun people fighting over it. However, one of the PCs sacrificing the Eye of Vecna to THE GOD OF JUSTICE? Not so expected.
In Unknown Armies, you can become an Avatar of an archetype by mimicking that archetype's classical behavior. One of those is The Fool, who can pull this off easily and walk away unscathed.
In Chrononauts, new players are the spanner. Plans in the game range from Gambit Roulette to "I win next turn as long as no one makes a minor change in 1914". New players will often meddle with history (even starting World War III), steal random historical artifacts, or kill the makers of said artifacts, to "see what happens".
In Exalted, beings that exist outside of fate are the ultimate Spanners from the perspective of the Sidereals. Since they cannot be detected, manipulated, or predicted by fate and fate-based powers, one of these beings can derail centuries of careful planning before the Sidereals realize that anything's amiss.
In Vampire: The Requiem, the Circle of the Crone has the position of The Fool, whose sole purpose is acting as this. When the Circle needs something stopped they send in The Fool and watch stuff blow up. Basically it's like "so, you're a rather spirited walking corpse, eh? Say, could you go check out what that Invictus bastard is doing downtown? Just do whatever feels natural, I'm sure it'll be a blast." Fools who survive long enough to gain some measure of respectability might make it to the rank of Trickster, for whom the purpose is basically the same except the Trickster actually has some idea of what he is doing.
In ''Tabletop Game/Champions 5th edition universe, The centuries long, multi-dimensional, wheels within wheels master plan of DEMON's secret master involves in it's final stages a literal Spanner in the Works. The precise timing that underlies the multiverse needs to be off just slightly for the rest of his plan to work and as such he has positioned a minion in the the metaphysical boiler room of creation with a $10 hardware store spanner. Now he just needs the stars to align....
In Grandia II, this role goes to Millenia. The Big Bad's plan requires that all eight pieces of Valmar be absorbed by Millenia so that Valmar's resurrection can be brought about and the Big Bad can merge with him. What the villain didn't count on was Millenia refusing to absorb the Horns after they merged with Ryudo, and then sealing them inside Ryudo rather than risk killing him. As a result, not only was Valmar's resurrection incomplete, but Ryudo eventually used the Horns as a weapon to kill the villain.
White Knight Chronicles: You are this for Bigger Bad Emperor Madoras. Or, rather, your custom character is. (The game features an incredibly robust character creation system, and it's an unwritten rule amongst the fandom that the avatar be based off of the player.) After sitting back silently during the story mode while (most likely, should you choose to play the game's plethora of side quests that require your avatar) being the most devastating character in combat, you are the only one able to take on Madoras in the game's bonus dungeon.
In Wrath of the Lich King, after Drakuru plays you for an Unwitting Pawn, you get to do this to his plans repeatedly.
Xenosaga: The entire plan of the Big Bad, that has taken centuries and all three games to complete, is undone because Allen can take a beating and look really pathetic while it happens. It's more awesome than it sounds.
A much more minor, but hilarious, example comes from Chrono Trigger. Ozzie, inept sidekick to the great Magus, was originally defeated by being dropped down a Trap Door. In his second coming, he's smart enough to make the switches instead drop the heroes down the trap door. When they come back, Ozzie's ready for anything... except for a not-so-random cat (if you get the regular ending, Chrono's cat(s) jump into the time portal), which wanders in, flips the wrong switch, and down he goes...
Revya and Gig, for all the multiple times they are Unwitting Pawns during the storyline, also become epic spanners: The demon path is basically you laying waste to the entire Gambit Pileup: Virtious, Thuris, Dio, Rashka and all the other manipulators' year-long plans are ruined by one free-roaming Omnicidal Maniac doing it For the Evulz.
Final Fantasy IV offers a minor example in the Dark Elf. A monster who messed up Golbez's plans to steal the Earth Crystal from Troia by snatching it first and running off to a cave where he rendered metal weapons unusable. Golbez works around this by sending protagonist Cecil to fetch the crystal in exchange for Rosa.
Believe it or not, Vaan from Final Fantasy XII fits this role very well. While he appears to be simply a throwaway character, he is the only one besides Ashe to see a loved one's image projected by the Occuria, in this case, his brother Reks. This implies that the Occuria felt that he would be a suitable and effective 'Plan B' to become the weapon which would destroy the Archadian Empire in revenge, should Ashe falter, probably due to his being both Dalmascan, and for, at the start of the game, harbouring resentment towards Archadia for the death of his brother. Instead, Vaan actually overcomes the illusions much more quickly than Ashe, ultimately providing a more virtuous role model for Ashe, as seen when he refuses to take revenge on Gabranth for killing his brother. He may just have been the push she needed to reject revenge at the story's climax.
It's not so easy to see because he's a minor villain, but the Dark Elf King Astos has been at his plot for years, then the Warriors of Light come out of nowhere, on a completely unrelated quest, and straight up murder him. And while it might seem like Astos spanned his own plot, his theory was that the artifacts he tricked the Warriors of Light into getting him would make him all-powerful, so revealing himself isn't stupid so much as Genre Blindness.
Dissidia 012 also retcons it that just one Warrior of Light, who is a Manikin. This guy is such a spanner that he manages to bring down plots in two worlds without even knowing his own name, let alone his ridiculously complicated origin story.
In La Pucelle Tactics the plans of both Noir and the spirit infecting Croix are foiled not by the Maiden of Light, but by Priere, a newbie priestess who is perhaps the least likely person you'd expect being a nun, though she has a mean right hook and an even meaner set of legs.
In The Force Unleashed, the cunning plan created by Vader and Palpatine fell apart due to one bit of (bad) luck and one unforeseen relationship. The accidental wiping of PROXY's primary programming resulted in a Heroic Sacrificewhen Vader tried to kill Starkiller, and the emotional connection between Juno and Starkiller meant she came back and rescued him after Vader's failed attempt to kill him. Starkiller himself then became the spanner when he went after Vader and Palpatine, saving the rebel leaders. The end result is that Palpatine and Vader ended up creating the Rebellion that eventually defeated the Empire. Oops.
In Mega Man ZX, the cornerstone of Master Albert's plan to become a god involved gathering the world's Model Ws and combine them into Ouroboros. However, aside from shitting out Mavericks like an Ominous Ring Of Land, it has no way to prevent an airship from crashing into it and inserting a war party intent on shutting him down. If he had a few more Model Ws, he could have sprung for mounted anti-air... too bad the Guardians (and Mega Man Model ZX) got to them first (and subsequently destroyed them). The thing to note is that this is a Spanner entry because Albert's plan would have been more complete had they not been destroyed; everything else went just as planned.
Actually the better example is how a pair of excavators accidentally awaken Grey, Master Albert's back-up body. After witnessing what Model W can do, Grey ends up saying no to Master Albert's plan and becomes instrumental in his downfall. Ashe also counts since in her storyline, she encounters Model A by accident and singlehandedly ruins Albert's plan as well.
Albert even notes that her appearing was entirely unpredicted, and the amount of trouble she caused for him surprised most people directly involved.
Team Fortress 2 - While the Spy's entire job is to be the Spanner in the enemy team's works, random chance tends to play this role in otherwise evenly matched teams, as nobody can account for that stray lucky critical hit.
Fear 2: Project Origin has this happen to the player instead of the villain. Terry Halford comes up with a telesthetic amplifier that could give the player character, Sergeant Beckett, a decent chance at exploding Alma's head, and then Genevieve Aristide waltzes in, shoots any dissenting voices, and changes the amplifier's settings in an attempt to trap Alma. Note that Halford, who has a far better grasp of the math and science of the amplifier, has already stated very emphatically that Aristide's plan simply will not work and Aristide is essentially a desperate idiot trying to make reality conform to her wishes. Her pitiful attempts only make Alma stronger, at your expense.
King's Quest VI: Heir Today, Gone Tomorrow: Grand Vizier Abdul Alhazred has been running an Evil Plan very successfully on the Land of the Green Isles. Gain the trust of the king and queen and their approval to marry the princess then arrange the kidnapping of the princess and kill the king and queen, making sure he's in charge. Spread rumors and encourage in-fighting among the islands to bring the kingdom to the brink of civil war after stealing their sacred treasures yourself and crippling the only non-magical means of transport so they can't visit each other and check. As the final piece, arrange a grand sham wedding to the princess and kill her after the wedding night. It's running like clockwork, and then the Prince of Daventry shows up with a mad crush on the princess...
King's Quest VII: The Princeless Bride: Malicia has kidnapped the Fairy Prince Edgar and the Troll King of the Volcanix Underground, then transformed and brainwashed the former to impersonate the latter and cause a volcanic explosion that will destroy the Fairy Kingdom of Etheria. But she didn't realize that her new accomplice harbored feelings for a certain Princess of Daventry and that his now morally deficient state would cause him to kidnap her, causing her mother to chase after her... and things kind of fell apart after that.
Speaking of Sierra, the Hero in the Quest for Glory series tends to be this at times as well, either by intent or accident. Basically all of Quest for Glory IV was a result of the teleport spell used to snatch him at the end of Quest for Glory III dumping him in the wrong place in Mordavia.
Sierra loved this one, because much of Space Quest ran on this as well. Roger Wilco had just the good (or bad) luck to send the best laid plans careening into a cliff.
The first game? The Sarians had boarded the Arcada, stolen the Star Generator, and slaughtered all of the crew...except Roger, who had been taking an unscheduled nap in the broom closet.
The second game? Vohaul had his cloned army (granted, they were an army of insurance salesmen) ready for launch, but having his Mooks sent to kill Roger in an attempt to prevent him from stopping it (after all, Vohaul bankrolled the Sariens), led to Roger showing up and wrecking the plan anyway.
Third game? Roger was just good enough at Astro-Chicken and just bored enough to find and investigate the secret message from Pug's captive programmers.
Fourth game? Once again, Vohaul tries to pre-emtively kill Roger from screwing up the plan. But the timely arrival of Xenon's time-traveling La Résistance sends that into a tailspin.
Fifth game? Captain Quirk was above suspicion, captain of the Confederation flagship. No one was going to investigate some toxic waste dumps on a few backwater planets. They all but laughed the ambassador representing those backwaters out of the room when she tried to call attention to the problem. But then a mouse chews up some wires in the academy computer, Roger gets accidentally promoted to Captain, and guess who finds out about Quirk's lucrative side business in transporting tetragenic sludge?
Sixth game: Stellar the the spanner. Sharpei wanted to commit Grand Theft Me on Roger, figuring he wouldn't be missed. But Stellar steps in at the last minute, makes a Heroic Sacrifice, and Roger decides he's going to rescue his pal. Sharpei and her plan did not survive.
Fire Emblem 7 has Kishuna and Hector. The first one ruins a Smug Snake magic user's trap for the heroes via his Anti-Magic powers, the second is such a strong support for Eliwood that Nergal has to admit that his mere presence throws his gambits around Eliwood off.
The Big Bad of Last Scenario had such a good plan going until Phantom woke Ethan up early. Then Ethan just happened to run into an Unwitting Pawn and warned him not to trust an agent of said Big Bad. Things kinda fell apart from there.
This is basically all the Boss does in Saints Row 2. The Brotherhood originally planned on bringing in a shipment of weapons to take over the city with. Boom, the Boss steals it all. Then they try to manipulate their friends out of jail. The Boss just kills all of them when they get out. The Sons of Samedi see their plans to sell drugs ruined by the Boss when he kills all their dealers, and when a lieutenant tries to trick the Boss into walking into traps, the Boss kills them too. Vogel, meanwhile, planned to manipulate everybody into killing each other so he could take his place at the top, which worked fantastically when he was getting the Boss to kill people instead of getting people to kill the Boss.
Raphael Sorel, the French fencer from Soulcalibur II, unintentionally saved the world and Siegfried (here in his guise as the Big Bad, Nightmare). Raphael appears as Nightmare's opponent in his Destined Battle. When Nightmare defeats Raphael and is about to finish him off, Siegfried is somehow roused out of his suppression by SoulEdge/Inferno and begins a Battle in the Center of the Mind, rendering his body immobile. Seizing the opportunity at hand, the injured Raphael takes his rapier and delivers a mighty thrust to Soul Edge's eye, weakening the soul of the sword and allowing Siegfried to fully regain control of his body. At that point, the holy sword Soul Calibur appears to aid Siegfried, who then plunges it through the evil sword again. This causes both swords to fall silent (having been locked into a state that rendered both dormant), setting up the plot of III and Siegfried's character progression to that of The Atoner. Raphael's reward? He gets infected by Soul Edge's malevolent influence (he unwillingly passes this on to his adoptive daughter Amy as well, when she tends to his wounds) and becomes some sort of vampire.
In Advance Wars 2, despite Andy having a spanner himself, It's Hawke's turn against Sturm that really destroys his plans.
In the chapter 1 finale of Dragon Fable, the Big Bad is undone by two spanners. The Bacon Elemental Orb (which no one but Cysero knew even existed until that moment) combined with Cysero's time machine gives the hero and his dragon a fighting chance in the Final Battle. Just when it seems like even that wouldn't be enough to save the day, the villain's host body Fluffy reveals his weak point to the hero and his dragon.
Grim Grimoire has the The Archmage preparing to break free from his Soul Jar and take the philosopher's stone and menace the world, and powerful devil he entered a pact with named Grimlet is about ready to break from being sealed inside the old wizard Gammel Dore. Neither of them could possibly predict that both of their plans would screwed up by one Cute Witch named Lillet Blann caught in "Groundhog Day" Loop.
Assuming the player never discovers the Dossier revealing the Thalmor's role in the civil war, the Dragonborn can become this for the Thalmor, who are running a long scheme to weaken their enemies by goading Ulfric Stormcloak into rebelling against the Empire and making sure the civil war goes on for as long as possible to drain both sides' strength. The Dragonborn can end the war with one of two outcomes, both of which are bad news for the Thalmor.
In the Dark Brotherhood (Assassin's Guild) Questline, the Dragonborn can also be this for Astrid. The Dragonborn is the Dark Mother's unwitting chosen one, which screws over Astrid's position as the "family don."
And the Ebony Blade was locked safely away in Whiterun, from champions who would be tempted into killing their loved ones and confidants to empower a stupid sword and go homicidally insane... until the Jarl's son became a talkative ass from the sword spirit's rumors. The Dragonborn figures out how to steal the sword thanks to the boy blabbing about the "woman behind the door."
As of BlazBlue: Continuum Shift Extend, Terumi's master gambit pileup for his goals remained untouched up until Makoto Nanaya fell into one particular timeline by accident and interfered with events key to his schemes - the end result being utter annulment of all of his plans in that timeline, and she got away with it with help from Rachel Alucard. As a result, from Continuum Shift proper onward, Terumi's plans for Makoto have changed from "keep away from Tsubaki and Noel" to "outsource to Relius or kill on sight".
This is actually a reconstruction. Makoto's Spanner status comes from the fact that she never knew Noel never existed in this timeline up until Terumi's plans had already fallen apart. Other than that, she had proven through the entire story that she was capable of picking up on the world around her and using the intel she accumulated to formulate and rebuild her plans on a whim - one slip of the tongue by Terumi was enough for her to, with help from Bang's keen-eyed disciple, figure out that Relius was luring her into a trap and adjusted her plans just enough to defuse the problem and keep at work. Had she known about that missing fact any time before she ran into Tsubaki, she could have outgambitted Terumi instead.
In Kingdoms of Amalur: Reckoning, The Fateless One is this to everyone and everything because he/she is Immune to Fate. Furthermore, since he/she was a Blank Slate with no memories upon revival it's impossible to predict just how he/she will react in any given situation. Everyone who tries to manipulate the Fateless One for their own evil plans fails miserably. Even Tirnoch, the being that allowed the Fateless One to exist in the first place, underestimates what she created and pays for it with her life.
In a way, this trope can apply to the other protagonists of the Pokemon main games themselves. The villainous team's plans basically get ruined because of a person from a small town began their journey at the same time they launch their plans.
In Max Payne 3, Max was the perfect Unwitting Pawn. A dumb gringo who'd been steered into making many enemies, he would be hopelessly lost at best, soon dead at worst. Unfortunately for the villains, good cop Da Silva showed up and gave him direction, and their plans started falling apart soon afterwards.
In Devil Survivor 2 the Main Character can be this for Yamato and/or Polaris by choosing another faction when Yamato probably would have won if not for the Main Character's interference and possibly convince Alcor to help kill Polaris and end Polaris' system.
In Dawn Of The Dragons, the dragons' long-running scheme to reconquer the world is derailed by two spanners. The first is a crazy wizard who wakes them up too early because he wanted to use them in his own scheme for world conquest. The second is the player character, an ordinary farmhand who frees an unborn dragon from his evil parents' influence and assembles an army alongside the newborn.
The modern day Sonic the Hedgehog games seem to have given Dr. Eggman these moments as he'd be close to victory, then something happens that turns everything on its head. For Sonic Adventure, it's Chaos not taking Eggman's crap anymore. For Sonic Adventure 2, it's that the Eclipse Cannon had a Doomsday Protocol that activated when all the Chaos Emeralds are gathered in it.
Tron 2.0: The F-Con CEO had been planning his takeover of Encom and invasion of cyberspace with Datawraith mercenaries for twenty years. (It's impled he was Edward Dillinger from the first film, out for payback after being forced out of Encom in disgrace). Things would have gone swimmingly...if Alan Bradley hadn't been arguing with his son Jethro at the time F-con's thugs burst in the room and kidnapped him, and if Jethro hadn't been standing in just the right spot for Ma3a to kidnap him via Shiva laser.
Kristoph Gavin's ultimate scheme in Apollo Justice: Ace Attorney was meant to be the murder of Vera Misham via a poisoned postage stamp depicting the magician act Troupe Gramarye. However, the girl is such a huge fan of the Troupe that she saves the stamp for seven years; her father eventually uses the stamp instead and dies. The flaws in Gavin's plans run deeper than this, but this is his only apparent mistake; everything else is because of Phoenix Wright's meddling.
Larry Butz also counts, owing to his tendency to do unreasonable things that end with him stumbling onto vital evidence. In the first game, he was coincidentally returning a boat he had been using at precisely the right time to overhear a gunshot, in the third he shirked his work as a security guard when the villain's plan relied on him being at his post so that he would hear the noise of a panic button, rush into the room and arrest the wrong person and then in a later case his choice to wander around at night in the cold leads to him witnessing a number of things he wasn't meant to.
In the fourth case of Justice For All, the lead that helps Gumshoe and company track down Shelly de Killer is, of all things living and not living, Matt Engarde's cat, who meows at the end of a transmission from de Killer.
In the third case of Justice For All, Acro's plan to murder Regina was to call her to a specific point where he would drop a heavy weight on her head. Problem is, the note he secretly planted on Regina began with "To the murderer...". Due to Regina being very naive, she didn't think the note was for her, and posted it on the circus's bulletin board, where her father saw it and responded to it in Regina's place. Then you realize that Acro wanted to kill her because her naivete led to his brother being put into a coma, and he should have known that she would never realize the note was meant for her...
In the final case of Ace Attorney Investigations, the shenanigans of Larry Butz and Wendy Oldbag, of all people end up helping Miles Edgeworth put away the seemingly untouchable Big Bad. Larry, by accidentally breaking the Samurai Spear, forces the staff to have the Steel Samurai use a different move instead, thus enabling Edgeworth to realize that Alba was not at the Steel Samurai show while he was killing Manny Coachen. Wendy Oldbag takes the box of "Rising Sun Dogs," which turns out to have a drop of Alba's blood on it, and combined with Alba's wound, proves that he was injured while killing Coachen.
Larry gets to do this again in the manga. Simply by being arrested on suspicion of killing Bright Bonds (based on calling him and demanding that he get out of his ex-girlfriend's life), he derails the killer's plan to use him as an alibi, because she gets called in to prove his alibi, and through contradictions in her testimony, gets implicated as the murderer.
In Virtue's Last Reward, Dio aspires to be one for the AB Game. Too bad for him that not only is his presence expected, it is in fact required.
YOU can potentially become this in True Love Junai Monogatari, if you choose to romance Ryouko Shimazaki. Ryouko is actually an Idol Singer (under the identity of Sonoko Takahashi) and has apparently retired, much to the sadness of her fans (and the profit of her sponsors). Her manager and brother/parent figure, Tadaaki, actually had her fake her retirement to pique the interest of the fandom, then make the greatest come back ever — and since Ryouko is both his younger sister and his ward, she cannot stop said plan. But after she befriends the MC, he inspires her to try speaking up and be herself, rebelling against the stage brother who controls her life and actions...
Also, if you choose to romance either Mayumi Kamijou or Yumi Matsumiya (who can't be romanced if you don't finish Mayumi's path first, you will also ruin Mikazaki's plan to blackmail Mayumi into sexand will give Yumi enough proof to get Mikazaki himself humiliated and kicked out of the school.
In the Homestar Runner holiday toon "A Death-Defying Decemberween", Homestar announces to one and all that he's going to sled down the Steep Deep - a vertical cliff face - and Strong Bad catches The Cheat surreptitiously helping Homestar bury a mattress at the foot of the alleged slope. Of course, Strong Bad being Strong Bad, he moves the mattress expecting Homestar to maim himself on impact...but the next day, when Homestar sleds down the Steep Deep, he makes a perfect landing. As it turned out, the mattress was full of "hammers, broken glass and candy canes sucked down 'til they're all pointy"; the whole thing was a ridiculously elaborate (and painful) scheme for Homestar to get out of having to spend Christmas with his girlfriend Marzipan's parents, one that Strong Bad successfully sabotaged (even if the end result wasn't quite what he had been expecting).
Kurama: "...so I believe Hiei's superior speed would be the best choice for this fight." Hiei: "Well Kurama, your plan sounds good except for one fatal flaw." Kurama: "What? What are you talking about? My plan is foolproof!" Kuwabara(screen-shift): "Here kitty kitty!" Kurama: "I stand corrected."
In the Sluggy Freelance story arc "GOFOTRON Champion of the Universe," Zorgon Gola has a pretty nice Gambit set up where he pretends to be an Omnicidal Maniac, so that the heroes will sacrifice themselves trying to prevent a chain reaction that would destroy the universe, leaving him free to take over the Punyverse after their deaths. What he didn't count on was Torg, Riff, and Bun-Bun accidentally teleporting themselves into the Punyverse. They end up hijacking a vital piece of the heroes' Combining Mecha (the crotch). Without this, the heroes have no way of pulling off their Heroic Sacrifice, and Zorgon Gola, along with the rest of the Punyverse, is blown up. Oops.
Quentyn Quinn in Tales of the Questor is a subversion. Instead of stupidity, it is a sense of honor and desire to help that make him accept quests which almost always indirectly monkey-wrench SOMEONE'S plans, generally without even knowing it. Examples:
His very decision to become a Questor—paired with the discovery of an old contract—puts his home village in peril. The contract stated that the previous Questor was to retrieve some artifacts in exchange for land on which to build a village. The problem: the old Questor failed to do so, and now that there's a new Questor, a political faction intends to use that contract to get Freeman Downs repossessed and thus out of the picture. Quentyn's decision? Retrieve those artifacts, knowing that if he dies in the attempt, the law ensures that the contract ends with him. He knows nothing about the true scheme at work, and the opposing faction has no idea what to do now. The legal counsel sent to deliver the bad news summed this trope up rather perfectly:
Roy Greenhilt of The Order of the Stick might qualify for this in one early instance, despite being a very intelligent person, simply because he does something so very unexpected. Xykon honestly doesn't expect the heroes to stop him (and rightly so, as he has an army of monsters, 10 or more levels on the strongest PC, and a monster strong enough to send Paladins flying by LIGHTLY TAPPING THEM). He has three characters immobilized, two more being stalled by monsters, and he had JUST shattered the leader's ancestral sword. Confident that the battle is as good as over, he starts to call out the aforementioned monster to finish the heroes off. And then Roy goes and tosses his bony ass into a body-destroying gate that holds an Eldritch Abomination at bay. The Big Bad is out of commission.
Elan seems to fit this more than Roy does. Daigo at one point wonders if he's more useful the less he knows what's going on, and Durkon suggests that "He has Ignorance as a class power source." In 691, Roy lets him wander around the desert in the hopes that he'll stumble over something. WHICH HE DOES!
The majority of the plot of Demonology101 consists of Raven and her friends following this trope.
One of the characters in the Blip is this to God's master plan.
The main cast of Drowtales are Manipulative Bastards and Chessmasters whose plans often wind up crashing into each other, but so far the biggest example of this trope is Ragini, a child slave who survived the massacre at the Val'Sharess tower and was able to keep said Queen alive for a year, and then later become host to her aura and escape. If it weren't for her things in the story would be much, much different, and the full extent of her spanning potential is just beginning.
A smaller version in Girl Genius —British spy/Gil's man Wooster just caused Dolokov's plan to corrupt/distract the Jaegergenerals while Wulfenbach forces destroy Castle Heterodyne to go thoroughly off the rails.
In Season 9 of Survivor: Fan Characters, Prescilla/Bonnie has been controlling the entire game and has managed to take the two weakest players to the finals with her while tricking everybody into believing that she's just a sweet, innocent girl who couldn't POSSIBLY be an evil mastermind and gloats about how she has a 100% chance of winning the day just before the final Tribal Council. Then Cherman, the little robot who has been a Butt Monkey for most of the game, reveals that he has a recorder he used to record Bonnie's Evil Gloating about how her only regret is that she didn't do more to hurt others. Your reign was nice while it lasted, Bonnie.
When Tipping Forties LP'd Tales of Symphonia, they managed to go about 90 videos without using Presea, who they perceived as The Scrappy. The point was they hated the flatness of the character and refused to involve themselves with how stupid that part of the game was. Of course, they forgot the game was stupid (for that one instance, at least) and a scripted event forced them to use her. It nearly brought them to tears. And it wasHilarious.
In the Whateley Universe, this is Jade's purpose in life. She's screwed over at least four plans, simply by being there. See "It's Good to be the don", "Christmas Elves", "Christmas Crisis", and Ayla 7-6.
It's extremely rare for this to not happen at least once per game in Comic Fury Werewolf. It also happens intentionally a surprising amount, considering.
Boomer, a Mad Bomber from the SCP Foundation storyline "Game Day", tends to carelessly leave dangerous explosives in random places around his hideout, to the point that it's not really possible to plan a safe approach to it.
"Lighter Than Hare," where Yosemite Sam (hilariously cast as a space alien) thinks that Bugs Bunny would be so foolish as to take on his indestructible tanks and unbeatable robots. All Bugs needs is a simple dynamite stick to easily beat back the challenge.
Various cartoons pitting Bugs Bunny vs. Wile E. Coyote, with the Coyote hoping to use his brains and elaborate scientific contraptions to capture and make mincemeat out of Bugs. Only it is his intended prey that foils the Coyote — and all he usually needs is a dynamite stick or to pull a switch.
"Wet Hare" is built on this trope. Here, Bugs' opponent, Black Jacque Shellacque, thinks he's got Bugs' next moves figured out by building dams of various strengths. All Bugs needs to do is trick Jacque into pulling a tiny rock from near the dam's base, or (after Jacque mistakes a phony shark for a real one(!)) getting the shark to ram another dam, to softening Jacque's inhibitions by distracting him with a single dynamite stick ... before sending a huge raft of explosives crashing into the dam to beat him back. (Bugs eventually gets rid of Jacque once and for all by tricking him into trying to blow up the federally controlled Grand Cooler Dam!)
Spider-Man and His Amazing Friends: Miss Lion, Aunt May's dog, in "Seven Little Superheroes." By tagging along with Peter Parker-as-Spider-Man on a trip to a mysterious island, she is able to foil The Chameleon's scheme to separately kill off Spidey, Iceman, Firestar, Captain America, Doctor Strange, Namor the Sub-Mariner and Shanna the She-Devil, particularly when she can sniff out evil drones disguised as various X-Men.
Ed, Edd n Eddy. The Eds' scams might have worked if Ed weren't so stupid to break them. You'd ask why Eddy tolerates the lump. Actually he doesn't tolerate him, he once kick him in the crotch just for misplacing his magazines.
Pinky is the downfall of pretty much all of Brain's schemes.
Subverted in one episode, where Pinky convinces all of the world's leaders to hand over control of the world to Brain on a silver platter, only to have Brain himself torpedo the plan.
There are several occasions where it's Brain's own oversights that doom his plan (the Jeopardy episode where he blew the last question due to it being a pop culture one, as an example. And Pinky is completely obsessed with the show the Final Jeopardy question is based on. If only Brain listened to him...
There's also the episode where Brain builds a machine to calculate exactly what common factor keeps causing his plans to go wrong. Sure enough, the catalyst is not Pinky, but Brain himself. Which raises the question of how Brain could fail to be a common factor in all of his own schemes.
Lampshaded in a Kids' WB! promo for the show, in which Pinky confesses:
"I'm not really that stupid. I purposely sabotage Brain's plans because if he ever succeeded, the show would be over, wouldn't it?"
In the Rugrats episode "The Bank Trick", Tommy and Chuckie mistake an ATM machine at the bank for an M&M machine and think the bank is actually a candy store. They don't find any, but they do inadvertently trip a security alarm and expose two bank robbers posing as federal bank inspectors.
Deedee from Dexter's Laboratory is almost always the one to ruin her little brother's scheming. Dexter turns this into Flaw Exploitation when he learns about his Arch-Enemy Mandark's crush on Deedee, and turns her loose in his lab.
Coop of Megas XLR does this constantly. Prime examples: he once destroyed a planet-eating monster by firing an EMP-missile-turned-fridge packed with Pop Rocks and Coke, and in another episode he destroys the Glorft mothership by accidentally beaming his slushie onto one of its control boards. Not to mention all of the times he's done it to himself and other good guys.
Bullwinkle, of Rocky and Bullwinkle, is an idiot whose sheer stupidity endlessly frustrates villains Boris and Natasha.
Inspector Gadget is a perfect example, as he often inadvertently helped Penny and Brain solve the cases through his clueless bumbling:
Penny and Brain are trying to prevent a nuclear missile from being launched at Metro City, but it's taking too long and the countdown has started. We then see Gadget wandering through another part of the MAD complex, where he damages some important equipment. This aborts the countdown and gives Penny and Brain the time they need to disable the missile.
Gadget is swallowed by the robotic Stock Ness Monster that Dr. Claw has turned loose in an important lake, and he proceeds to start messing around with its inner workings. This causes the monster to go haywire and start swimming around the lake at random, which allows Penny and Brain to get inside of it and take control themselves.
Gadget is surrounded by MAD agents in a cave, and tries to get away by using his Gadget copter. The copter's rotor blades become stuck in the cavern ceiling and cause Gadget to start spinning out of control. His Gadget arms and legs start flailing around, pummelling the MAD agents and knocking them senseless.
Some MAD agents are about to spray a toxic wood-rot formula over a forest from an airplane. Penny tries to use the woodrot formula to knock a tree onto the runway to keep the plane from taking off, but the spray nozzle clogs and she only sprays enough to weaken the tree. Along comes Gadget flying in the Gadget copter, and he crashes into the weakened tree just as the plane is about to take off, knocking it over and stopping the plane dead in its tracks.
And then there was the time Gadget was in one of Dr. Claw's undersea bases, and opened the seal that kept the place from being flooded...
Gadget is pursuing a MAD agent in the Gadgetmobile, and Dr. Claw arrives in the Mad Mobile to try and stop him. In trying to catch the MAD agent, Gadget repeatedly activates the wrong gadgets, unknowingly spraying Dr. Claw with laughing gas and then firing a missile at the Mad Mobile that leaves it trapped in glue and unable to stop Penny from taking control of the MAD agent's vehicle to finally capture the villain.
MAD has taken control of the new supercomputer that the Metro City Police Department uses to coordinate its efforts to fight crime. When Gadget tries to use the computer to contact Chief Quimby, he thinks that the machine is broken and tries to "fix" it. He inadvertently wrecks MAD's connection over the computer, and then alerts the police to what's going on, enabling them to catch all the MAD agents who were committing crimes while the police were distracted.
Gadget is in a Spot the Imposter situation with a MAD spy who's disguised exactly like him. No one can tell which Gadget is real until our hero stands next to Chief Quimby. Gadget's mallet activates by itself and bonks Chief Quimby on the head, and the dazed Chief immediately orders that the other man be arrested, since the one who hit him is obviously the real Gadget.
Used explicitly and spectacularly in the second C.A.K.E.D. episode of Codename: Kids Next Door. At the episode's climax, Nigel responds to the Delightful Children's five-person-unison speech about having planned every last detail by declaring that they forgot one thing: "MY CRAZY GIRLFRIEND!"
The Flying Brains of Futurama intend to collect all the information in the universe and then destroy it, using their telepathic powers to keep anyone from stopping them. But they didn't count on Philip J. Fry, the one man in history too stupid for those powers to affect.
Archer: In the two-part season 3 finale, Pam and Cheryl stow away on the spaceship, causing them to miss their trajectory for Mars and land on the Intrepid. Later, when Pam, Malory and Cheryl are held up on the ship, Pam decides to lock out the bad guys to stall for time until Archer rescues them. her plan is foiled by Cheryl, who opens the door and lets the bad guys in so she can become their Martian Queen.
Ron Stoppable. And when the bad guys remember to account for Ron, out pops Rufus.
In one episode Dr. Drakken goes so far as to leave Kim's father out of his gathering of geniuses-to-turn-idiots. When demanded to tell him why, Drakken states its because he realized kidnapping him would just get Kim's attention. He didn't count on his scheme being right next to the ranch run by Kim's uncle and her, Ron and her father being there on vacation at the same time, though.
The South Park cast finally decide to ignore Cartman after he eats the skin off the KFC chicken. Cartman thinks he died as no one communicates with him and Butters is the only one who can see him. A psychic explains that he is stuck on Earth to deal with a crisis. After learning of a hostage situation, Cartman believes he is the only one who can help. Believing he can't be harmed, he freely walks into the Red Cross Center and pretends to be a ghost while moving items around. The robbers don't know what to do and the police move in once Butters frees the hostages. The two are credited for saving the day "armed only with the weapon of confusion".
Avatar: The Last Airbender : Long Feng's plan to trick Team Avatar into leaving Ba Sing Se would have worked out perfectly if Smellerbee and Longshot hadn't just happened to cross their path and inadvertently reveal that Jet had been brainwashed.
While Candace Flynn isn't evil, her attempts to show her mom what Phineas and Ferb have done would be more successful if Dr. Doofenschmirtz's crazy inventions didn't inexplicably cause their projects to vanish.
King Sombra in the season 3 premiere of My Little Pony: Friendship Is Magic was only thwarted because Spike went along with Twilight and helped her get around all the traps that King Sombra had placed to guard the Crystal Heart. It's also ultimately Spike who delivers the Crystal Heart to Princess Cadance after Twilight gets trapped and can't deliver the Heart herself.
Queen Chrysalis in the season 2 finale after drawing Twilight's suspicion. She manages to keep her charade going thanks to Twilight's inability to give her friends solid evidence that something's wrong, but in trying to get rid of her, she leads Twilight right to the real Cadance. This leads to Cadance reuniting with Shining Armor, and their love blasting Chrysalis and her minions right out of Canterlot.
Cedric in the second season of W.I.T.C.H.. Will had a Batman Gambit set up so that Prince Phobos could gain a hold of the power Nerissa stole and, when he would attempt to wander into Candracar, he would lose it and they would get it back. Will (nor anyone else) had expected Cedric to get tired of Phobos' crap and eat him.
As deranged as some of Zordrak and Urpgor's schemes to steal The Dreamstone are, some of them almost do the job were it not for Sgt Blob and his soldiers' blundering. The odd occasion Urpgor plays an active part in a mission he usually proves to be just as detrimental.
Mirta from Winx Club manages to derail the Trix's scheme to trick Bloom into thinking she was descended from the Ancient Witches. Later, when she's a pumpkin, she spots the Trix's nightmare monster and warns the Winx, allowing them to prepare for it.
One episode of House of Mouse had Pete attempt to shut the house down by sabotaging the thermostat, driving the customers away. Just as Pete barges in to demand Mickey fork over control (as its said that, if there's no people, then there's no show, then Pete gets back the house), both Pete and Mickey find out there's someone still there: Hades, who is so very used to the heat. Pete finally drives Hades out by flooding the place. Just when he finally thinks he's won, Ariel shows up.
Many a Scooby-Doo villain has seen their counterfeiting, diamond smuggling, theft, fraud, or other criminal scheme foiled by "those meddling kids!" Since the Scooby Gang usually rolls into town without any knowledge of a ghost or a monster running around beforehand, one assumes that some or all of those schemes would have worked beautifully if the Gang hadn't unwittingly injected themselves into the situation.
Greece might qualify as If they hadn't held off the Italian invasion that required Germany to help them, Operation Barbarossa might have been started earlier, and the invasion of Russia might have succeeded.
Even before surrendering to the Allies in 1943, Italy was probably as much a hindrance as a help to the Axis war effort through opening new fronts and then calling for German help. The desert war was another example, starting when Italy tried to invade Egypt from Libya and promptly collapsed to the British counterattack until Rommel arrived … which, of course, meant Rommel wasn't on the Eastern Front. At a diplomatic conference before the war German Foreign Minister Joachim von Ribbentrop sniffed to British Foreign Secretary Anthony Eden and Winston Churchill (then only an MP), "In the next war Italy will be on our side." Churchill replied, "That seems only fair. We had them last time."
Between May 10th and May 14th, the Dutch shot down 43% of German planes deployed over the Netherlands, including 51% (220 of the 430) Junkers JU-52 transport planes. This may or may not have influenced the Battle of Britain or a potential airborne attack on the UK. The Dutch proved to be a tougher nut to crack than the Germans imagined in general. The Germans expected the Netherlands to fall within a day, which caused Hitler to issue his 11th war directive which was "The resistance of the Dutch army is stronger than expected. It must be broken as soon as possible."
Hitler himself. If he hadn't meddled with everything, thereby wasting resources (like the V2 — great for propaganda, useless in warfare), the Germans might actually have had a chance. And that's not even counting that brilliant idea of attacking Russia. Late in the war (1944-45) this became fairly explicit: The British cancelled plans to assassinate Hitler because they believed any replacement leader would be more effective, extending the war.
The Philippines. Even though bad luck and the Idiot Ball had ruined any chances the Filipino-American force had of successfully turning back the Japanese, the defense of Bataan and Corregidor ultimately fucked up the Japanese war plans by holding on much longer than any other Asian country aside from Nationalist China (almost FOUR whole months, from the attack on Pearl Harbor to the surrender of Corregidor. Malaya, Singapore, Hong Kong and the Netherlands East Indies had capitulated earlier). And then, there's the various guerilla movements harrying the Japanese occupiers and providing the Allies vital intelligence.
During the Battle of Midway, the Japanese were not defeated by any cunning strategy, but because a group of lost dive bomber squadrons suddenly stumbled upon the Japanese carrier group while it was rearming all its planes, making them extremely vulnerable. This group wasn't even traveling together, it was three separate squadrons that all stumbled across the carriers almost at once, right after the Japanese fighters protecting the fleet had been drawn off by the (failed) attack run on the fleet by Torpedo 8, a squadron from the USS Hornet that had become separated from their fighter escort by poor visibility.
Taffy 3 at the Battle Off Samar. A small collection of tiny ships due to their audacity managed to stop a force of massive battleships in their tracks. They did this with the torpedoes on the destroyers being the only effective weapons available to the American fleet. It also helps that the Japanese ships were Glass Cannon's that aren't as tough as the American ships.
One possible ally to the Axis could have been Spain, but it decided to remain neutral, in part due to its close Relations to neighbour Portugal, which decided to also remain neutral. Therefore, by not doing anything, Portugal kept the Axis form gaining another ally, Spain.
In World War I, Germany intended to go through neutral Belgium in order to attack the French. It probably would have worked if not for the Belgian resistance delaying them. Worse than that for the Central Powers in the long run, Britain had no intention of entering the war until Germany invaded Belgium. If Britain had remained neutral, the US likely would have, as well, and it's quite probable Germany and Austria-Hungary would have won if their only sizable enemies were Russia (which collapsed in revolution), France, and Italy. Even Italy was a toss-up, as it effectively waited to see which side would give the best offer until 1915. Italian designs on Austrian territory probably gives the edge to the Entente, but without the British involved the Central Powers might have had a chance.
On a slightly lesser note, the Portuguese intervention in the War, on behalf of Britain, started with the taking of great part of the German fleet, which was stationed in Portugal. This meant that not only they could not properly attack Britain, they could also not protect themselves against the US.
Santa Anna's plan to finish off the Texas Revolutionaries at San Jacinto on April 22, 1836 was ruined by Sam Houston's decision to attack first a day earlier despite the Mexicans outnumbering the Texans 1,400 to 900. Santa Anna also sealed his own fate by diverting too many of his soldiers and failing to post lookouts while his army rested — not to mention supposedly getting seduced by the "Yellow Rose of Texas" Emily Morgan. The Mexicans surrendered to Sam Houston's assault after just 18 minutes of fighting.
If you're doing user interface design, one of the best tests is to hand the software to an entirely untrained user. They will ferret out all kinds of little bugs and quirks because they'll choose utterly ridiculous but nonetheless logical ways to use your application.
Similarly, video games. Less skilled gamers will find those accidental holes in the wall before experienced gamers, who aren't, you know, accidentally running into walls all the time.
Developers of the perl programming language have stress-tested new versions by having it parse /dev/random as input. Bugs that had resulted in segmentation faults were discovered this way. Throwing shit at the fan and see what happens is a fairly common way of stress-testing, usually known as fuzzing.
This trope describes much of the history of economics. Even the most brilliant economic models and philosophies can break down because people are neither strictly rational nor in possession of "perfect information."
Professional poker players can sometimes be thwarted by novices and amateurs, who make plays that no professional would be stupid enough to attempt and end up short-circuiting the professionals' expectations. This also is true for billiards players.
A Canadian fraudster used a complicated scheme involving disappearing ink and forged cheques to embezzle thousands of dollars from the banks he held accounts at. It's difficult to explain briefly, but it involved him writing a cheque to transfer funds from an account he held at one branch to the account he had at another bank. The scheme depended on his cheques being cashed at the first bank on a Friday, then the ink disappearing over the weekend, and processed at the second bank on Monday, which would give him more money in his first than was deducted at his second. Unfortunately, on one occasion the fraudster had the bad luck of dealing with a rookie teller who didn't know how the cheque was supposed to be cashed, and didn't start working on it until Monday. The boss noticed the discrepancy, accused the teller of writing the information wrong, and called the police on her. The police discovered that the check had actually been written partially in disappearing ink, and the fraudster was quickly nailed.
Another fraudster reprogrammed a bank's computers to periodically shave ten cents off every account and apply it to the account under the last name on the list alphabetically. All went well until a Mr. Zydel opened an account and got confused as to why his bank balance kept inexplicably increasing. Zydel did the honest thing and reported it to the bank, who investigated it and had the fraudster arrested.
A couple of fraudsters decided to make large amounts of fake $20 bills, and in turns they bought food with those at McDonald's and other stores which worked so well they earned several thousands of dollars from the exchange money. Around half a year later they went to Las Vegas and gambled for a week without getting noticed... until one day a woman who had the weird habit of ripping the upper-left edge from the bills noticed the paper was white inside (they did not use enough ink). The couple wanted to leave but was quickly taken by security and in their apartment they had 4 four more boxes filled with fake money.
A version from The American Civil War: on the second day of the Battle of Gettysburg, Confederate General Lee devised a strategy to attack both flanks of the Union army simultaneously in hopes of overwhelming them before they could be reinforced by the center. General Sickles from the Union ignored orders and moved his corp out in front of his fortified position to engage the enemy, a move of staggering stupidity that got his corp massacred. However, this delayed the main Confederate assault on that flank by several hours, destroying any chance the Confederacy had of launching the attacks simultaneously. As a result the attacks went on separately, and were beaten back separately. This in turn led to Lee gambling desperately with Pickett's charge the next day, a disaster that effectively destroyed Confederate momentum in the war. While people often "what if" this battle, it is pretty generally agreed that Lee did not do a very good job of commanding the battle in general, and there were far, far more problems than this one.
Thomas Blood's plan to steal the British Crown Jewels failed only because the elderly caretaker's son came back on leave from the navy at precisely the right moment. Seriously.
This happened to Tennessee in their 2010 college football match against LSU. UT had the game won when LSU stupidly didn't have a play ready for a third-and-goal with seconds remaining and no timeouts, resulting in a botched snap that looked to end the game. But UT trumped it with their own stupidity in a last-second personnel change that resulted in Two Many Volunteers on the field (four guys came on while three ran off, and then one of the three ran back onto the field). The illegal participation penalty forced the down to be replayed (American football games can't end on a defensive penalty), giving LSU time to settle down and organize the game-winning play. You can watch the last moments of the Dumbass Miracle here.
Related to sports, Bill Simmons' "Levels of Losing" have as the seventh "The Monkey Wrench", where a bad decision by either the coach or the umpire costs the victory.
Political parties in America are this, believe it or not. The government was set up so that all three branches—executive, legislative, and judicial—would be more or less in opposition to each other. The rise of political parties put an end to that in a hurry. Indeed most of the founding fathers were completely against political parties, but viewed them as a necessary evil that would naturally arise despite their best efforts.
In Major League Baseball, you have the Steve Bartman story. In 2003, the Chicago Cubs were a very good team whom many picked to win the World Series (and finally break the Curse of the Billy Goat). And during the National League Championship Series against the Florida Marlins, it seemed like this would be a reality. The Cubs built a 3-0 in Game 6 and ace pitcher Mark Prior was retiring batters with ease, working a three-hit shutout through the first 7 1/3 innings. Then Marlins shortstop Luis Castillo hit a deep foul ball that sailed towards the edge of the stands, a ball that Cubs outfielder Moises Alou had a shot at catching for the second out of the inning. Instead, a Cubs fan (Bartman) interfered with the play by reaching out his hand to catch the ball. This angered the Cubs (Alou was very visibly frustrated after the play), who complained to the umpire for a fan interference call. The umpire ruled against them, stating the ball had left the playing field when Bartman touched it (if it hadn't, Castillo would have been called out). The most upset was Mark Prior, who completely lost his focus and began giving up hit after hit. He was taken out of the game and replaced, but the relief pitchers, also unfocused, gave up nothing but walks and hits. The Marlins scored 8 runs in that single inning, and ended up winning that game, Game 7, and eventually the World Series that year.
On 20 June 1789, King Louis XVI of France decided to have the meeting room for the Third Estate deputies of the Estates-General redecorated, but forgets to tell the deputies this. They subsequently turn up, find the room locked and believe that it's the start of an attempt by the king to prevent them from meeting. This rumour spreads until on 14 July, believing an attack is imminent, several thousand Parisian citizens arm themselves and march to the place that holds Paris' gunpowder stores - the Bastille.
The 9/11 attacks could have been much worse if not for a major spanner. Four planes were hijacked; one was probably intended to crash into the White House. However, the terrorists did not take into account the passengers attempting to retake the plane. The aircraft crashed a hundred miles or so from its target, and although lives were lost, it would have been far worse if the White House had actually been hit.
In a story from Not Always Working, some video game store employees inadvertently thwart a shoplifting scheme in the process of setting up a prank.