The main protagonist Nadja is this. 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. Nadja not only gets away and finds the reports, but is the one who manages to give them to the authorities and soon Hermann is thrown in jail for attempted murder and his own massive debts.
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.
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.
Earlier in the story, Ymir became one to the Marleyans' plan to destroy the Eldians in Paradis from the inside, as she ate the Warrior group's leader, Marcell Galliard, and took the Jaws Titan for herself, leaving Bertolt, Reiner and Annie without their leader.
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.
In Basilisk, Princess Oboro may not be able to fight, but she derails Big Bad Tenzen's plans thrice. First, she tries to interrupt a fight between Koushirou and Gennosuke but accidentally causes Koushirou to blind himself with his own powers, limiting Tenzen's physical actions since Koushirou is his Number Two. Second, she seals her own Piercing Eyes to avert using her incredibly lethal powers against her clansmen again, but also preventing Tenzen from using said powers for his sole benefit. And third, her eyes are unsealed when Tenzen is reviving himself after Gennosuke kills him, which allows her to undo Tenzen's Immortality and kills him for good. And in the end she also derails Ofuku's plans via commiting a Heroic Suicide instead of killing a completely defenseless Gennosuke, sending her into a hugeVillainous Breakdown.
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 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...
The cat 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 (mostly 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 as he geasses her into becoming a Genocidal Maniac and then has to shoot her dead to stop her, and where he abandoned his followers for his kidnapped sister shortly afterward which greatly weakened their position, 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, probably because he's smart enough not to misuse a power that only works once).
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 subordinates who were very worried about doing something like that, including Cornelia (who had never considered such a thing, but went along with the idea anyway)... and 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 couldn't bring himself to kill a woman who is unarmed and, at that moment, no threat to him. Bad decision, there.
The Compilation Movie series essentially tones down her role as this. While she still tries finding evidence about Lelouch, her inolvement with Shirley is changed to having Deithard be the one to shoot her and cause her amnesia in order to retrieve her evidence. Even when Schneizel in this version reveals C.C is the source of Lelouch's Geass, the Black Knights show more reluctance towards betraying him by wanting him to answer their questions first before passing judgement. It's not so much her or Ohgi this time as it is Schneizel and Kanon by having their men be the ones that try to gun down Lelouch before the Black Knights could get their answers.
One of the biggest would have to be Princess Euphemia. When Brittanian occupiers were murdering Japanese civilians by the dozens and pretty much enslaving those who survived, 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.
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.
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. The missing information, of course, was that LIGHT HAD HIS OWN NOTEBOOK HE ALREADY USED!
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.
Shidou, a fairly stupid shinigami, performs this twice:
He 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 basis for his suspicion, and Light would not have handed over Misa's Death Note to Mikami. 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?
Den-noh 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.
Heroic Gambit Example: Between them, Heiji Hattori and Ai Haibara screw up a plan 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 him. She showed up just as the plan was going his way, and 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 relies on whether Touma's Trauma-Induced Amnesia will be undone or not.
The Mystery Train arc has the Kaito Kid forcibly drafted into this role by Conan in a plot to convince the Organization that they killed Haibara. When everything's over, the only one in the Organization to realize they were duped is also the only one who was guaranteed to keep quiet about it.
In the Whole Episode Flashback of manga chapters 921-924, Shinichi himself was one of these at the age of four. In his attempts to befriend a just as young Ran in his new daycare, he briefly caught a weirdexpression in the face of the daycare worker in charge. He mentioned that as well as other details in front of his parents in what seemed to be just a display of childish jealousy... this but allowed his Guile Hero dad Yusaku to realize that the teacher and his Ill Girl wife were planning to kidnap Ran and raise her as the Replacement Goldfish of their runaway daughter. Yusaku alerted the police (including Ran's Action Dad Kogoro), had the wife and her brother/accomplice arrested, convinced the teacher to turn himself in willingly in exchange for being reunited with the missing daughter, and Ran was ultimately saved — all because Shinichi made his first deductions right in the perfect moment.
The Red Ribbon Army is a worldwide military organisation planning to use the Dragon Balls to Take Over the World. Too bad they didn't expect to be dealing with a 12-year-old boywith a stick after the Four Star Dragon Ball left to him by his deceased grandfather. In fact, Goku may not have been inclined to attack the Red Ribbon Army's headquarters... until they sent the hitman Tao Pai Pai to kill the guy who happened to have the ball Goku wanted. This meant Goku had to make amends to the guy's son, and that meant he now had to get all the balls to revive the dude... including the ones the Red Ribbons had already collected. Their base is now a target.
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.
Gohan and Krillin become this to both Vegeta and Frieza on Namek. First, Gohan steals the Dragon Ball that Vegeta took and hid. Second, Gohan and Krillin teaming up with Vegeta against the Ginyu Force kept them alive long enough for Goku to arrive and save them, taking out the last major force on Frieza's side. Then, their friendship with the Namekians gave them access to the Dragon Balls, allowing them to wish Piccolo back to life, bring him to Namek, and deny Frieza and Vegeta their chance at immortality.
Vegeta ends up being a spanner at the climax of the Cell Games arc. While Gohan is in a beam struggle with Cell, all of the other fighters seem helpless to even budge Cell, and it seems that Cell is set to kill Gohan. But, Vegeta hits Cell with a blast from behind that's just strong enough to make Cell briefly lose his footing, distracting him. This allows Gohan to unleash all of his remaining power, destroying Cell down to the atomic level, finally getting rid of him.
King Yenma and Fortuneteller Baba are this in the Buu arc. Had they not brought back Vegeta to fight Super Buu, Goku wouldn't have anyone to fuse with, and would have remained completely outclassed by Super Buu.
In Dragon Ball Minus, Frieza recalls all the Saiyans to their home planet so he can kill them all at once in a month's time. He does this because he fears the Saiyans rebelling and becoming Super Saiyans. This alerted Bardock and and his wife, Gine, so they sent their youngest child away. That child grew up to become the Super Saiyan who kicks Frieza's butt.
Jaco the Galactic Patrolman ends up being this towards Champa's team in the Champa arc. Vegeta had suspected something wrong when, during Goku's match with Frost, the Saiyan is taken down so easily. However, during Frost's match with Piccolo, Jaco spots Frost slipping his right arm inside Piccolo's stretched and wrapped arm. When Frost defeats a now-poisoned Piccolo and declared winner, Jaco objects and tells the announcer to check his right arm. Frost had a needle implanted inside his right forearm coated with poison. Frost is disqualified and disgraced while Piccolo becomes the real winner. Even more, Jaco risked being obliterated on the spot for a possible lie to expose this.
Later on in Super, Cell does this posthumously. In the beginning of the Future Trunks arc, Goku Black destroys Trunks' time machine, rendering the heroes unable to return to the Future Trunks timeline... but what Black didn't know was that Cell had also time-travelled to the present (since his Future Trunks had already destroyed the Androids and prevented him from achieving his perfect form) and left his version of the time machine behind. The heroes found it, and Bulma kept it, providing them with a backup to follow him with.
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 — because all of it makes him immune to Izaya's manipulations. 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.
The first cour of Endride revolved around a) Prince Emilio wanting to kill King Delzaine, b) his friends trying to dissuade him because they need answers from Delzaine, c) the Ignauts trying to get Delzaine to peacefully step down while fighting off his enforcers, the Truculent, and d) Shun trying to get to Babylon so he can use the Babel device to return to his world. Come the mid-season climax, instead one of the Truculent, Ibelda, slaughters all his teammates, kills Delzaine himself, and jumps into the Babel device, meaning absolutely no one got what they wanted, and a whole new set of problems were created besides.
In the second part of the Explorer Woman Ray OVAs, Ray uses a crystal pendant on the Ord temple — which uses controlled light beams as a means of somehow controlling the weather — in order to stop bad guy Rieg Vader from controlling it.
In the Battle of Fairy Tail arc of Fairy Tail, Erza's false eye is this. Its existence halves the effect of any ocular magic used on her. When Erza is petrified by Evergreen using an ocular attack, the eye allows her to break free and defeat Evergreen.
It turns out Scar's older brother was the first to figure out Father's Ancient Conspiracy (more exactly: he was the first person to notice the transmutation circle around the country and began piecing together what's going on; the raid on his country confirmed his suspicions, and he quickly devised countermeasures for it). And he did this before the series even starts! He may have died to save his younger brother before he could execute the plan but he played a major role in defeating Father, by allowing the heroes to fight back. And even more: by transmuting his own arm to a mutilated Scar plus imbibing it with the right kind of alchemy, he not only saved his younger brother's life (at the high price of further damaging his mind and soul), but helped him become the one who ultimately took down Fuhrer Bradley aka Wrath, one of the most dangerous Homunculus.
One 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, there's Ling Yao and his entourage, who can sense Homunculi and just want the secret of immortality; May Chang, who uses alkahestry instead of alchemy (therefore she lacks its weaknesses) 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.
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.
Smith sending a helicopter to shoot down the truck delivering GSX to Team Sugo gets sabotaged by a 14-year-old boy on a motorcycle who gets unhurt and capable or driving the car to safety.
A sabotage on Hayato before the Niseko GP gets ruined when Ohtomo Johji, a veteran friendly driver, happens to be out to practice that night as well.
On the Niseko GP, Bootsvorz's first attempt to crash out the car gets ruined by Hayato's sudden pit entry for a set of rain tire. An unexpected rainfall that follows causes him — running on slick — to spin out.
His second attempt also gets ruined because a random car gets out of a mud trap and blocks the gap between to two machines. The heavy rain causes him and the said car to crash each other out.
Smith's personal attempt at taking out both Shoemach and Bootsvorz gets spoiled when Hayato, driving GSX into a shortcut, jumps out of nowhere and scares him, indirectly leading him to his death.
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.
Gundam: Nine times out of ten, bad guys in 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 thought that 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, the Colony wouldn't have been destroyewd and Amuro would have NOT been in the right place and time to hop into the Gundam that his father built...
Domon Kasshu. Suffice it to say, he's a thorn in the side of anyone who ever has a plan in this series (including, sometimes, people on his side). Examples include showing up in Shinjuku just as Master Asia is putting his Evil Plan into motion and being in love with the person Ulube selects as the core unit for the Devil Gundam: via an Anguished Declaration of Love, he managed to break through Rain's brainwashing and release her, with The Power of Love fueling the God Gundam's ultimate attack that let them destroy the Devil Gundam forever and ever.
If it wasn't for Rain "borrowing" the Rising Gundam during the Finals, Wong's plan to kill Domon and install Allenby as the Apocalypse Maiden in charge of the Devil Gundam might well have succeeded. Instead, Rain took Allenby down WITHOUT killing her and then her father removed the DG cells from her body, which later let Allenby help Domon free Rain from the Devil Gundam.
Even more: Rain's father Dr. Mikamura used to be in league with the bad guys. When his belovedRain found out and called him out, Mikamurarealised how terrible all of this was and decided to turn tails on the villains, which led to Rain taking Allenby out of Wong's plans. And then, when Rain was seized as the Apocalypse Maiden instead, a fatally wounded Mikamura managed to alert Domon and his friends of the upcoming danger and, in his dying throes, released the capsule containing Domon's cryogenized father, which deprived Ulube from his other hostage and gave Dr. Kasshu the chance to help as well.
In Mobile Suit Gundam SEED Destiny, Durandal ends up throwing a spanner towards himself and his Destiny Plan by repeteadly trying to assassinate his political rival and the local Guile Heroine, Lacus Clyne. Had he not decided that Lacus had to die, then Lacus's boyfriend Kira and Kira's friends in the Archangel crew wouldn't have gotten back together to fight back and 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.
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. Ultimately, he threw in his Trump Card, a Brainwashed and Crazy Meijin Kawaguchi III and his Exia Dark Matter, causing the duo to defeat him and also expose the truth of those particles.
Mashita's twin brother, the leader of the Gunpla Mafia, also has his plans wrecked by Reiji. He had planned to take over the Yajima Stadium and defeat the Battlers of the 7th World Gunpla Battle Tournament (the ones who ruined Mashita) to take control of Gunpla Battles all over by demoralizing people. He ends up taking down Sei, the Meijin and Mao by manipulating their Plavsky Particle amount and holds back China, Nils and Ricardo and prepares to finish off Sei when Reiji suddenly teleports in, take control and uses the Arista Aila had to repower Sei's suit and defeat the leader.
Mobile Suit Gundam: Iron-Blooded Orphans: Iok Kujan, through sheer, unadulterated incompetence and stupidity, completely derails Tekkadan and McGillis's plan to deal with the Mobile Armor Hashmal. Twice. First he activates the Mobile Armor by showing up in a Mobile Suit, despite precautions taken by the other side not to bring Ahab Wave-powered machines to the dig site precisely to prevent that from happening. Later, after Tekkadan has taken careful measures to draw the Mobile Armor into an ambush site near a city they evacuated, Iok shows up to (futilely) attack it, drawing its path away from the evacuated city and towards a still-populated agricultural plant, which is subsequently destroyed.
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.
Jeudi and, in the anime, 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 reveals her knowledge of the Alpine Rose song when she's being taken away (by singing it in the anime and or leaving a handwritten version of the lyrics in the anime), 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 using 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...
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.
Lupin III Part 5 has Lupin's ploy to fake his death to get away from assassins nearly fall apart due to Zenigata running up and "rescuing" him. If it wasn't for this, Lupin would have successfully pulled his plan off right there and then, but instead is forced to trek across the desert and have both of them, as well as Ami, nearly die of thirst. (He does successfully manage to pull of the plan eventually, though, after some improvisation.)
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 (or at least what's left, since 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.
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.
Midori Sugiura from My-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 located and rebooted. More importantly, she lets her keep her memories of Alyssa, deciding to trust in that. More than the right decision considering this allowed what seems to be the spirit of Alyssa to activate Miyu's strongest mode and smash the pillars containing the souls of those sacrificed, with the side effect of freeing the previous Crystal Princess...
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. He 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 also 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 Sharingan 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 the Third Hokage's death), a failed attempt to foil Akatsuki's plans as The Mole and also 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.
Kabuto threw a spanner in Tobi's plan summoning the real Madara Uchiha. He did made Tobi's plan far more complicated as he never intended to resurrect Madara in the first place.
Ironically, Tobi himself became a spanner in Black Zetsu's plans with his reluctance to resurrect Madara and a wish to take control of the Moons Eye Plan since Black Zetsu wanted Indra in place of Madara in order to complete the plan, forcing him to use Kabuto to revive Madara as a zombie in the first place. Also his HeelFace Turn and his wish help Team 7, becoming a spanner to Kaguya's plan to get rid of them and ultimately becomes the cause of her defeat and re-sealment.
Nodoka, Kazumi and especially Chisame derail Kurt Godel's Thanatos Gambit, because when Negi goes One-Winged Angel on him after he reveals what happened to his Doomed Hometown, Nodoka and Kazumi hold him in place, and then Chisame slaps him back to sanity.
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 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 wants to live is allowed to live. Or SOMETHING...
Mireille Bouquet may 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.
Most importantly, Mireille's mother Odette's Last Request was... for Kirika, her own soon to be killer and later Mireille's aforementioned partner, to take care of Mireille. A last wish that, many years later, Kirika would ultimately follow to the letter — ruining Altena's desire for a Chloe/Kirika Noir match.
Luffy and his crew have a tendency to do this wherever they go. Captain Kuro, Crocodile, Eneru, CP9, Shiki (from Strong World), and Z (from Film Z) 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 Hody 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 nine Straw Hats to fight, Hody 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.
Monkey D. Luffy in particular:
Luffy 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 beating three of the Seven Warlords of the Sea (and earning the respect or allegiance of the other four, and allying with yet another who proved to be The Mole after more were added to the roster), 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.
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?!
Luffy may be the biggest spanner among the Straw Hat Pirates, but Sanji deserves special mention as well; his skills as a tactician often prove to be the linchpin to the Straw Hats' success. Enies Lobby shows his shrewdness in this twice. First, he lures a bunch of Marines and Agents into the back of the train Puffing Tom, then slips past them while they're distracted and detaches their cars behind him, stranding them and dealing with almost all of the remaining Mooks and one One-Man Army without having to fight them. Then, when the Buster Call arrives at Enies Lobby, he disappears for a while. When he returns, it's revealed he'd been looking for the controls to the Gates of Justice so he can close them and throw the Buster Call ships into disarray.
Much earlier, he shows off his skills in the Alabasta saga. First, all the way back in Little Garden, he happens to intercept a call from Mr. 0 and, pretending to be Mr. 3, tricks Baroque Works into thinking Vivi and the Straw Hats are all dead, meaning they don't have to worry about constant pursuit for a while; plus he intercepts an Eternal Pose to Alabasta, meaning the Straw Hats could leave right away rather than wait a year for the Log Pose to reset. Later, in Alabasta itself, all of the other Straw Hats except Chopper have been captured by Crocodile and trapped in a room that will soon be submerged. He uses a bit of trickery to lure Crocodile and his Dragon out of the building and lure them far enough away to give him time to sneak in and set all of them free. He even taunts Crocodile with a note saying "Mr. Prince was here" for when he comes back and discovers that he's been outsmarted.
In Skypeia, the first time Eneru shows any real shock was when he discovers Luffy's rubber body is shockproof. The second time, though, was after he KO's Sanji, but his Parting-from-Consciousness Words hint he's still ruined Eneru's day...by sabotaging the Ark Maxim!
As of 862, Sanji might have performed his biggest spanner act ever... simply by calling his bride Pudding's Third Eye"beautiful". Up until then, the Ax-CrazyBitch in Sheep's Clothing Pudding was fully willing to shoot him dead with a Hand Cannon... but since she had been bullied all of her life for that same Third Eye, Sanji's kind phrase shook her to the core and sent her in a sobbing Villainous Breakdown. This meant: she couldn't bring herself to kill him, Sanji managed to perceive and dodge additional threats in their surroundings (like an attack from Pudding's very badass Cold Sniper older brother Katakuri)... and then Luffy (and some clones of his) got the perfect opening to begin crashing the wedding, undo Big Mom's Kill 'Em All plans regarding Sanji's family, and put the plan to assassinate Big Mom herself in movement.
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 the Clown also qualifies in that same arc. His desire to grandstand and show off to a world audience led him to capture a video Transponder Snail, 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.
At the beginning of their journey into the New World, the Straw Hats evade Smoker this way. They were originally planning to go to Raijin island, and Smoker knew they were going there because it was the most dangerous of the New World's first three islands, fitting Luffy's adventurous personality and reckless tendencies. However, due to a timely distress call and pure happenstance, they end up going to an island the Log Pose didn't even register. Thus began the Punk Hazard arc, where the crew accidentally stumbled upon another long term villainous plot and Government Conspiracy, that they inevitably wreck. Again. And this time, they have a live audience to witness it all over the New World.
Going to Punk Hazard destroyed many villains' ongoing plans. It not only derails Doflamingo's 10 year rule, but also devastated Big Mom's, Shogun Orochi's as well as Kaido's plans. Big Mom could never have laid hands on Sanji if not for the trip to Dressrosa, which in turn bolstered Luffy's own reputation as a prospective Emperor when Luffy went charging in to the rescue. Orochi and Kaido's enemies were there in Punk Hazard itself. As they rescued Kin'emon and Momonosuke, they also revitalized Wano's resistance movement against the tyrannical Orochi, and Momo himself is revealed to be the son of Oden, the transcriptor of the Roger Pirates. Also they accidentally saved Zou from being destroyed by Jack, only because Kin'emon and Momo stated it as their destination. The red Poneglyph in Zou in turn provided the most important clue to the route to Laugh Tale. It's safe enough to say both Wano's resistance and Luffy's quest would have failed completely by accident if Luffy didn't decide to step on the most dangerous island to report on the distress call.
Smoker's obsession with catching Luffy has made him into a spanner for a quite number of villains as well. The moment where the Straw Hats evaded him upon entry into the New World led to Smoker following them to Punk Hazard. The presence of both him and Tashigi later proved to be pivotal to the Straw Hats and Law taking down Caesar Clown, as it was Smoker who got Law his heart back from Vergo, while his own heart was used to trick Caesar into accidentally killing his subordinate Monet, right before she was about to blow up the island to kill everyone except for Caesar, who, being a Gas Logia, would've been the only survivor. He also proved to be a spanner for the World Government's propaganda practices. He was absolutely disgusted by the Medal of Dishonor situation issued to him and Tashigi at Alabasta, and confessed as much to the newly conscripted Admiral Fujitora, whose goal/dream is to one day end the Warlord system, admitting the only reason he didn't reveal the truth is because he lacked the power and influence to make people believe him. This inspired Fujitora to delay his report after Doflamingo's defeat and expose to the surrounding islands both Dressrosa's current state and their true saviors, the Straw Hat Pirates and Law, preventing his superiors from trying to cover it up. And that, led to the abolishment of the Seven Warlords System.
In the Dressrosa arc, Donquixote Doflamingo hatches what looks like the perfect trap to destroy the Straw Hats with. See, this guy isn't your average villain; not only is he an extremely skilled and powerful Chessmaster and Manipulative Bastard, he's also smart enough to know that underestimating Luffy's crew would be a fatal mistake. However, outside factor after outside factor helps the Straw Hat Pirates out, rendering it all moot. The Tontattas, whose people were enslaved by the Donquixote Family, provide intel on the location of the Smile Factory and the identity of the power-user maintaining control over the country's Living Toys. A familiar face from the past shows up at the colosseum, and he probably wouldn't have without the tournament Doflamingo created to trap Luffy, to switch places with Luffy, allowing him to attack Doflamingo's castle undetected. But the biggest spanners came from Doffy's own crew: his officer Violet, who maintained a façade of loyalty since the time she was blackmailed into joining, falls for Sanji's chivalry and switches sides. Giolla, on the tail-end of her capturing the Thousand Sunny, lets her guard down long enough for Brook to pull a You Are Already Dead. Bellamy helps Luffy escape the colosseum because Doffy decided to have him killed. And most of all, there's Sugar, the one turning people into toys. Her being knocked unconscious is the only way for the guise over Dressrosa to be lifted, but no one can get near her. Usopp is captured, but instead of turning him into a toy like the others, she feeds him a grape she believed to be poisoned... which was actually hellishly spicy, and the resulting Jump Scare / Nightmare Face causes her to faint in shock. Consequently, Doflamingo's precious Masquerade is shattered, the country sees him for the evil person he truly is, and he's forced to take the most drastic measures that he can.
There's also the fact that in the resulting battle, five of the eleven remaining One-Man Armies in his crew are taken down thanks to pirates that came to the colosseum due to the tournament.
It turned out that Sugar herself commited quite the mistake regarding her plan... Due to how her powers work, once she makes somone into a member of the Living Toys group, Sugar must manually create a contract with the victims, forcing them to follow every order given at that point, against their will. She did not do that with her first victim... who turned out to be the son-in-law of the former King, the extremely badass gladiator Kyros. As a result Kyros, now the toy Thunder Soldier, managed to retain his free will and do what he could to raise his not-turned daughter Rebecca, became the leader of an underground movement of rebels against Doflamingo's rule, and helped out the Straw Hats to overthrow Doflamingo.
In a non-Straw Hat example, there's Trafalgar Law. The Dressrosa arc, which escalated to involve nearly every major power in the world in some form — the Seven Warlords of the Sea, the Four Emperors, the Marines, the World Government, and the Revolutionary Army — wouldn't have even happened if it weren't for him in the first place. His alliance with Luffy really did "shake the world" as the papers said, as it put the Straw Hats in the path of the Donquixote Family, leading to all of the above, and the gradual destruction of Donquixote Doflamingo's criminal empire. The thing is, Law, much like Luffy in that regard, doesn't care about any of it. The ultimate purpose of all his actions, including continuing to live to this very day, for the last thirteen years, has been to kill Donquixote Doflamingo. If the world has to spiral into chaos just so he can kill the man who murdered the most important person in his life, then so be it... though it's revealed in some chapters that he also seems to want the aftershock he knows will follow Doffy's defeat. Luffy grants his wish for the most part: Doffy's not dead, but he's clearly ruined. So when the government kicks him out of the Warlords and puts him and Luffy on equal footing, he shrugs it all off as if to say, "So Long, Suckers!"
The aforementioned "most important person" in Law's life, Doflamingo's younger brother Rocinante, aka Corazon, not only allowed Law to become a spanner in Doffy's plans, but also was a spanner in his own right. By treating Law with sincere kindness when no one else would, Corazon reached for him and became his Big Brother Mentor, beating Law's potential Evil Mentor Doflamingo to the punch and restoring Law's lost humanity. Later, when Doflamingo obtained an incredibly powerful Devil Fruit (the Op-Op Fruit) Corazon forcibly fed it to Law instead, curing him from his lethal illness andgranting him other VERY good skills. True, Corazon was shot dead by Doflamingo for that (and for being The Mole for the Marines), but ultimately he had a spectacular last laugh since, as said above, Law ultimately became Doflamingo's biggest nemesis.
Lola from Thriller Bark. During Thriller Bark, she supplies Luffy with the shadows that allow him to pummel the giant zombie Oars, and though that doesn't destroy Oars, it does weaken him enough for the Straw Hats to finish him off. Then, before they part, she gives Nami a Vivre Card that apparently belongs to her mother. In Totland, it's shown that Big Mom is Lola's mother, and that card is full of Big Mom's Soul Power, allowing Nami to control the Animate Inanimate Objects also created by Big Mom. In fact, by leaving Big Mom's territory, Lola disrupted an Arranged Marriage that would have allowed Big Mom to become Pirate Queen!
In a larger scale, there's a woman named Sora. She foiled the plans of her husband Judge to experiment with their quadruplet sons, that she was still pregnant with, by taking a dangerous drug that undid gene modifications. While only one of the boys was spared being modifiedandher health was damaged beyond repair, which later killed her, it turns out the son that she did manage to save eventually escaped from his Big, Screwed-Up Family and grew into... none other than Vinsmoke Sanji, one of Luffy's True Companions, who's been helping Luffy become a bonafide example of this trope.
The Darkest Hour in the Wano arc comes when Shogun Orochi manages to head off the heroes' attempts to covertly build up a rebellion and raid Kaido's island base, Onigashima, by sinking all the ships they had planned to use to travel by water and the bridges they would have used to go by land the night before they were all set to gather. This happened due to the actions of a traitor in the good guys' midst passing along every scrap of information they could, including a last minute change of plans orchestrated by an ally just before his execution. What results is the Ninja-Pirate-Mink-Samurai Alliance arriving at their planned rendezvous, finding no one there, and the traitor revealing themselves in order to abduct Momonosuke. However, it's then revealed that the message that brought the main allies to this place meant something completely different. The meeting place was different than indicated and, because of the differences in distance between where they were and where everyone else was, everyone else was already in position long before the ships and bridges were demolished. Kin'emon, the leader of the samurai, is praised for such a brilliant piece of tactical misdirection, which exposed the traitor, prevented their actions from derailing their plans, and guaranteed the raid on Onigashima could proceed. The thing is, the only reason the allies were in the wrong place was because Kin'emon misinterpreted the message that changed their plans while everyone else didn't. Upon realizing this, Kin'emon can only stand there stone-faced as he realizes basically "Oh, that's what that meant..."
In an odd inversion, Ainz unwittingly messes with his subordinate Demiurge's plans to Nazarick's immediate benefit. By showing up in the Empire and giving a perfect demonstration of how overpowered he is (he willingly turned off his melee protections, used no magic and still wiped the floor with a giant, intelligent and PVP-optimized troll), Ainz gets the Empire to declare itself a vassal state of Ainz in the hopes that they'll fare better as his property than as his enemies. Demiurge (who sincerely believes Ainz is a master plotter more skilled than himself) is flabbergasted: he never doubted the Empire would eventually submit, but he expected it to require a long and bloody war of attrition over several months, and even then only after having subjugated the neighboring Kingdom and acquiring its resources. Of course this all went over Ainz' head: he simply cannot grasp that ordinary humans might find the godlike lich who can kill thousands with a single spell horrifying, and was slightly miffed that the Emperor was rooting for the troll rather than a fellow ruler.
Demiurge does it to Ainz all the time on a much lower scale: since Ainz' greatest fear is that his underlings find out he isn'tThe Chessmaster they think he is, he always tries to get the others to give their opinions on the plan (so he can learn what the plan is, being just as ignorant as the audience), and then appear to have planned it all along... only for Demiurge to laugh and explain that Ainz' genius runs far deeper than that, causing Ainz to scream internally as the others beg Ainz to explain the true plan.
Ringo Oginome pretty much tells Shoma Takakura that he became one for her "Project M" in episode 11. 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.
In Pokémon Adventures, 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.
Bewear of the Sun and Moon series usually evacuates Team Rocket just as they're about to get their ass kicked, though on some occasions it cuts in just as they're about to defeat the twerps. In some instances they become savvy to this, and one time even leave a distraction for Bewear so they have time to finish a more elaborate scheme.
Nebby's Teleport-happy antics also ended up jogging Lillie from her Trauma-Induced Amnesia and exposing Faba's scheming that he'd covered up for years.
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.
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.
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... in the direction opposite to what he wanted.
Reborn! (2004) has a mid-level mechanic incidentally named Spanner that performs a HeelFace 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.
In Red River (1995), 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 be a naive but very fast-learning Plucky Girl who decided to fight back to protect herself, then became the partner and 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.
Rosario + Vampire: In his first appearance, Gin tried to set Tsukune up to take the fall for Gin's own perversions in an effort to have Moka all to himself. The only reason it failed is because Kurumu was about to comfort Tsukune just as Gin intercepted him. Kurumu followed them and stumbled across a certain episode behind the school involving a digital camera and the girls' locker room. Afterwards, she rescued Tsukune from the vengeful locker room girls, confronted Gin on the roof, and presented a testimony that confirmed Moka's already-growing doubts about Tsukune's role in the incident.
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 the first, Aphrodite had devised a pretty good back-up plan shall he lose his fight with Shun: he anticipated the Saints would try and have someone keeping him busy while the other continued to run, so he planted his poisonous roses on the stairs behind his temple to act as Deadly Gas. By all rights, he had won with mocking ease, and it was about to work since Seiya fell for it in his despair and urge to go forward... But then, Seiya's mentor Marin showed up. As a Female Saint, Marin had to wear a Cool Mask — so she put it on Seiya, which allowed him to recover from the poison even when it poisoned her instead. As he wasn't breathing poison anymore and he saw his beloved mentor collapsed, Seiya gained enough physical strength and Heroic Willpower to destroy the roses, clearing his path to the top and towards the Big Bad. And by the way, Shun defeated and killed Aphrodite anyway.
In the second, Aphrodite apparently lost his first fight against Andreas Rize after being revived and was absorbed into the mighty Yggdrassil tree... but in reality, he had become this and was waiting for the exact moment to show it. When Loki-in-Andreas'-body got his hands on Gungnir and destroyed the Odin Robe, the tide was turned a the fallen Gold Saints returned, all because Aphrodite was immune to plant toxins-even Yggdrasil's- and was able to develop countermeasures, so the also absorbed and "killed" Gold Saints were only kept in a deathlike state but did not actually die. Aphrodite even pointed out Loki's fatal mistake to his face.
Senoo Kaori 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.
Also, 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.
In Sakura Wars: The Movie, the Rose Division's involvement in helping the Flower Division take back their theater and destroying the Japhkiel incubation facilities, as well as Ogami's return from Paris, ultimately put a dent in Douglas-Stewart's plans for making the Flower Division obsolete.
Mugen 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.
The Princess has a habit of messing up the plans of both the Demon King and the Hero in her quest for a good night's sleep while in captivity. She will frequently steal and/or damage items meant to be used or found in the course of the Hero's quest, rendering them completely useless. In one instance, the Demon King and his generals are discussing a series of items, tools, and weapons they plan to dispatch against the Hero, only for the Princess to be shown ruining each and every item they describe, and in one cast its guard, in the course of washing her bedsheets.
When one of the boss monsters comes back from being defeated in battle against the Hero, he complains that the items provided for him were all more useless than the last. While the first couple instances likely were the Princess, as evidenced by being inside of sacks made of Ghost Shrouds, monsters the Princess regularly hunts for materials. However, as the list goes on, one after another the Demon King's generals, and even the Demon King himself, realize that they were responsible for the items being useless, such as replacing a bag full of grenades with avacados out of fear the Princess would tamper with them, or a robot designed to resemble the Princess being redesigned into uselessness because the Princess complained. While she was only indirectly involved, everyone lets the boss monster blame the Princess than admit guilt.
The Future Gadget Lab in Steins;Gate, a literal basement (apartment) amateur lab in a cheap building is this to the world back and forth. Completely by accident they discover Time Travel which gives the Committee of 300 SERN division the ability to conquer the world, becoming a Spanner to the world. As they perfect the Time Travel method they become one to themselves by painting a giant target on their backs, in order to fix this they have to reverse what they have done essentially becoming a spanner for SERN and the Committee, and the best thing is, due to the timeline override SERN is non the wiser.
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.
Inspector Oblivious Mihoshi 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, Tenchi Muyo! Ryo-Ohki 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. He should know, since Mihoshi's meddling led to the release of Washu, whom he held hostagein a Pocket Dimension inside Kagato's Cool Spaceship that no one but Washu and Kagato knew of, and yet Mihoshi somehow got there with ease.) 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.
Washu, because the villains always tend to underestimate her, and in this case this lets her lock herself in her laboratory to decode the Big Bad's plans without anyone disturbing her, which she manages to do almost flawlessly.
The second spanner is Sasami, because Yugi got distracted by her desire to keep her out of her schemes since she's her Morality Pet. This was pointed out by The Dragontwice, and it never stuck.
The third and biggest spanner is Sakuya, who despite being Yugi's "shadow", in the end turns out to be more independent than she should've been, and ends up breaking through a Lotus-Eater Machine that keeps Tenchi away, snapping him out of it and letting him talk her down.
Tenchi the Movie: Tenchi Muyo in Love!
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.
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.
The plan to try and remove Kotetsu from the picture 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.
In the Makai Arc of Yu Yu 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.