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.
On 9-1-1, a few of the situations shown occur due to things getting out of control.
A worker at the firehouse and his bank manager brother in law hit on what looks like a good plan for a robbery. The manager uses a drug to create what seems to be an outbreak on a money truck driver to bring the cops in. He also drugs himself and then gets locked inside the vault. While the firefighters are busy inside, the worker poses as a hazmat agent to get the other driver out of the truck. He then steals the money and puts it in the fire truck, intending to get it out later.
The first spanner is that the manager double-crosses his partner, taking advantage of locked in to steal $6 million worth of diamonds. At the hospital, he drops a call to the cops on what happened. Thus, just as the firemen return, the cops show up to take the money before the worker can get it.
The manager then gets hit with this as he figured he'd have plenty of time inside the vault to get the diamonds. He never counts on paramedic Hen ending up getting locked inside with him. He's forced to drug her too but knows the rescue crew will be coming in faster than expected. He thus swallows the diamonds to smuggle them out...which ends up causing him a fatal case of internal failure later that night.
The owner of a failing diner decides to set the place on fire to collect on the insurance for his family. He figures by the time the fire is noticed and reported, it will have spread out to cover any signs of arson. However, his son had to go back to get his computer just as the blaze erupted with his mother calling 9-1-1 immediately. While both survive, the firefighters' early arrival exposes the arson and has the man arrested.
On Agents of S.H.I.E.L.D. Lincoln's severe wounding by James during the fight on the Zephyr put him in a situation where he sees that Daisy has the necklace that she'd foreseen being present in the explosion that kills someone on the team, which clues him into the fact she's preparing to take the warhead up in the Quinjet herself which then motivates him to do it instead, being the only one who has the power to fry the Quinjet's systems so Hive can't stop it. Had James not seriously wounded Lincoln, he wouldn't have had the conversation with Daisy.
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.
In Brooklyn Nine-Nine, Captain Holt and Sergeant Jeffords begin to secretly "CompStat" the precinct office in order to meet a paperwork deadline, by secretly manipulating the detectives in order to increase efficiency. Unfortunately, both of them forgot that Detective Peralta was out of the precinct while they were doing this; when he gets back to find Detective Scully moved to his desk, Peralta shoos Scully away to the break-room, which leads to a chain of events which sees Scully accidentally start a fire and bring the whole thing crashing down around them.
In the season sixth finale, Jake assembles a "Suicide Squad" of rivals Wuntch, Stentley and the Vulture in a scheme to take down Kelly. However, Stentley's sheer stupidity keeps messing things up. At one point, Jake comes up with a scheme to fake a kidnapping with the Vulture assuring him he'll give the case to "the two worst cops in my division" to ensure it won't get solved. Too late, Jake realizes that the Vulture only considers these guys "bad" cops because they're not friends with him and in reality they're very sharp detectives who nearly ruin the plan by finding the truth.
This trope has made the lives of the investigators on CSI 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.
A great turn comes in an episode of CSI: New York. A therapist plots to kill her husband and make it look like it was done by a disturbed patient who she then shot in "self-defense." When she arrests her, Stella notes how the woman's plan was good and might have worked thanks to the preparation. What she could never have counted on was that when she shot the patient, a pack of cops and CSI techs would be literally across the street investigating another murder, get there in record time and the woman was unable to properly clean up the crime scene.
In the Diagnosis: Murder episode "Till Death Do Us Part", two fiancés attempt to pull off The Perfect Crime and murder the bride's father on their wedding day. They succeed in killing their target, but fail to get away with it due to the unexpected interference of a dog.
Generally speaking, the TARDIS from Doctor Who spends her days throwing herself, the Doctor and their Companions at various intricate plots specifically so they can act as nifty spanners, just by dint of shuttling between crisis points, whenever they may be. "VWORP-VWORP-VWORP" is generally the sound of the Villain of the Week's ten-step plan starting to belch smoke.
In "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 "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 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 by accident.
"Human Nature"/"The Family of Blood": Psychic schoolboy Tim Latimer throws a loop into the Doctor's plans to hide from the Family of Blood until their short lifespans were up by turning himself into a human and hiding out for a few months. Tim's psychic powers, you see, let him "hear" the Chameleon Arch containing the Doctor's memories and Time Lord nature, leading him to steal it after being invited into "John Smith's" office for a conversation, and opening the watch alerts the Family to the Doctor's presence.
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.
In the first season, the Lannisters have a very careful plan for dealing with Ned Stark in such a way that he would be discredited, but the Starks would decide against going to war, and thus the succession for the Iron Throne would be secured... and then Joffrey has Ned publicly executed for shits and giggles, causing the North to declare war and making the Iron Throne look so insecure that Joffrey's uncles Renly and Stannis both decide to take their chance and challenge Joffrey.
On a smaller scale, part of the Lannisters plan was to take the three visiting Starks (Ned, Sansa and Arya) prisoner and use them a political bargaining chips. However, on top of Joffrey executing Ned, the much overlooked eleven year old Arya turns out to have both a swordmaster and the skills to fight back and manages to escape Kings Landing. The Lannisters are left with only Sansa, and for the rest of the series have no idea what Arya's up to or even if she survived.
The survival of Arya, Bran and Rickon means House Stark isn't quite as dead as many characters believe.
In the sixth season, Margaery devises an as-yet unrevealed plan to free herself and her brother Loras from the Faith Militant. Said plan becomes moot when Cersei elects to have the entire Sept of Baelor blown up, killing Margaery, Loras, their father Mace, the High Sparrow, and dozens of others.
Indirect example but the wildfire which Aerys Targaryen left under King's Landing was a spanner in the plans the High Sparrow had to establish the theocracy in Westeros and ultimately leads to his death.
Also in the sixth season, Littlefinger's long-running plan to make Sansa Stark his new bride and turn her into a puppet Queen of the North is suddenly dashed when Lyanna Mormount decides to shame the various Northern factions into naming Sansa's "brother" Jon Snow the new King of the North.
Ramsay to his own father. While Ramsay takes orders from Roose, he does things Roose doesn't order and in doing so screws up Roose's plans. For one thing, there's burning Winterfell and flaying the Ironborn, when it seems Roose's offer to them of safe passage out of the North if they surrendered was genuine. Not to mention that Theon's value as a prisoner became virtually non-existent after Ramsay castrated him; Balon has no use for a son who can't continue the family line. When Roose has Ramsay use Theon to have the Ironborn at Moat Cailin surrender, the plan almost collapses because Theon is so emotionally broken that he can't command respect from them.
Roose: Ramsay delivered the terms. The Ironborn turned on Theon as we knew they would. They handed him over, trussed and hooded, but Ramsay... well, Ramsay has his own way of doing things.
Thanks to the High Sparrow's manipulations, Tommen ruins whatever plans Cersei and the Small Council had to curb the power of the Faith Militant.
Daenerys to the whole Game of Thrones, as per Varys. The original plan was for Dany to marry Khal Drogo to provide Viserys an army to take Westeros in place of dragons, but Dany's assimilation into the Dothraki, her supplanting of Viserys in their hearts and minds, and then finally her hatching three dragons upset all of his and everyone else's schemes. Nobody expected or predicted that Daenerys would do the things she did or become the great conqueror and liberator she has.
Stannis Baratheon's arrival at the Wall completely derails Mance's invasion, but also throws a wrench in Roose Bolton's attempts to solidify the North under his rule.
Renly's rebellion was one of the key factors for why Westeros was divided in the wake of the Succession Crisis after Robert's death. Ned wanted a smooth transition to Stannis over the illegitimate children, yet Renly insisted that Ned leapfrog the line of succession and make him King for ideological reasons which Ned refused and in response, Renly, Loras and their contingent of soldiers leave the capiital. This prevented Ned from having proper allies at the key moment of Robert's death, forcing him to turn to Littlefinger and Janos Slynt instead, leading to his downfall. In addition, Renly declaring himself a King was a key factor in the Succession Crisis becoming a War of Five Kings, rather than a straightforward conflict between Joffrey and Stannis as Ned intended. Since Stannis hadn't yet received Ned's letter he didn't know of his claim until much later. Renly declaring himself King challenged the line of succession, as Robb Stark pointed out, which led to the North and Riverlands electing Robb as King in the North. Had it not been for Renly, the North would have probably rallied behind Stannis as Ned intended, and there would have been a proper alliance against the Lannisters.
Daenarys' invasion of Westeros begins with seemingly one-sided odds; with her, the Tyrells, the Dornish, and part of the Greyjoy fleet, along with an army of Dothraki and Unsullied, versus the Lannisters and a small handful of supporting lesser noble houses. Enter Euron's alliance with Cersei, where he destroys Yara and Theon's fleet and abducts Yara, kills or captures Ellaria and the Sand Snakes, and destroys the part of Dany's fleet at Casterly Rock. Euron manages to pretty-much singledhandedly save the Lannisters from losing the war.
Get Smart, of course; Maxwell Smart is more likely to defeat KAOS by accident than on purpose.
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.
The entire premise of the first season is that the Good Place architect is finding his carefully constructed paradise, tailored to perfectly meet the needs of each inhabitant, is suffering cataclysmic disasters. He knows that something is a Spanner in the Works but he does not know that it is Eleanor, who was sent to the Good Place by mistake and her actual selfish nature and behavior is apparently incompatible with the very nature of the place.
The above is actually a subversion: Eleanor and her three 'friends' have actually been in The Bad Place all along with the intent of them believing that they were in heaven while their insecurities and clashing personalities serving as an eternity of psychological torture. Those disasters were deliberately engineered by the architect to make Eleanor fear being exposed as a fraud. However, he did not count on Eleanor undergoing genuine Character Development and his entire plan goes Off the Rails when she selflessly confesses that she does not belong in paradise.
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.
Shinji Kido/Kamen Rider Ryuki is this trope's personification when it comes to Kamen Rider. He has an unholy ability to barge in at the least convinient moment and his The Determinator tendencies make him impossible to remove from the plan once he gets there. Also, being an Idiot Hero, he doesn't know any of this.
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).
This is why Nate has to be a master of Xanatos Speed Chess. All it takes is for, say, the mark to make a change to his daily routine or an unexpected phone call or a sudden visitor to pop in and it can throw the team's carefully planned scam off.
Sometimes, the spanner comes from the discovery the mark is more dangerous than the team thought. For example, their simple con on the corrupt owners of a Chinese laundromat goes awry when they find out the place is the front for the Triads. Likewise, a sting on a Russian businessman at his daughter's wedding is thrown when they discover too late the guy is connected to the Russian mob and half the wedding party are ruthless mobsters.
Another spanner can be the team's amazing ability to hit Gone Horribly Right on their jobs. In more than one case, they con the owner of a corrupt business into thinking it's in jeopardy, expecting him to sell it off to them...and instead the owner just decides to shut the business down and put everyone out of work.
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.
Nate: The perfect plan, it's got too many moving parts, too much that can go wrong. You have to expect the perfect plan to fail, that's what I do.
Also seen in "The Tapout Job"; Nate is pretending to be a fight organiser from a different state. The Spanner comes in the form of the mark's assistant's cousin. The assistant apparently gets suspicion and calls his cousin, in whichever state Nate said he was from, who "knows every fight producer in the state". The cousin confirms that he's never heard of Nate's alias. Cue Hardison's "I cannot hack a hick" line.
"The Rashomon Job" features the crew trying to claim that they were the one who stole a priceless dagger from a museum years ago. However, it turns out that none of them got the dagger in the end because this trope was in play. Everyone's plans collided with each other, resulting in the dagger quite literally falling into Nate's hand.
In Malcolm in the Middle, Malcolm's dad Hal is accused of being the mastermind of a money laundering scheme in his company, which was set up by his corrupt coworkers to make Hal the fall guy. Unfortunately, the culprits made the mistake of claiming they witnessed Hal doing the money laundering every Friday... except Hal has been skipping work on Friday for the last 15 years to go and have fun, with proof of his adventures.
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.
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 Oshin, Oshin's very meddling mother-in-law Kiyo plans to part her from her son Ryuuzo when she leaves Saga to work in Tokyo, by intercepting Oshin's letters and cutting off her and Ryuuzo's communication. The plan works very well when Oshin moves to Sakata, stops getting Ryuuzo's letters (since he doesn't know she has moved away) so she thinks he's ditched her and vice versa, until two people ruin this: Oshin's other prospect love interest Kouta (who writes to Ryuuzo to call him out on his "abandonment" and say he will help Oshin and her son Yuu instead, making Ryuuzo realize there's something fishy going on) and Ryuuzo's sister-in-law Tsuneko (who finds the remains of Oshin's letters, reconstructs them and shows them to Ryuuzo).
Power Rangers Ninja Storm begins with the villain ambushing all the ninja schools (ninja being the only ones who could stop him) and capturing the students. Only five people manage to remain: the Wind Academy's Sensei (who was not captured, but was effectively neutralized by being turned into a hamster), his son Cam (who wasn't captured because he wasn't a student), and three student ninja who decided to be good Samaritans and so were Late for School. Since they were the only ones with anything resembling the proper training, the three students ended up becoming the new Power Rangers under the mentorship of the Sensei and with Cam's help, and soon they start fighting back against the villain.
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.
Salem: John Alden's mere presence in Salem is screwing up the witches' preparations for their Grand Rite. Increase Mather later becomes this as he basically appoints himself the inquisitor of Salem.
Stargate SG-1: 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.
Seska's cover story in Star Trek: Voyager as to why she has Cardassian blood factors despite ostensibly being a Bajoran (she received a bone marrow transplant from a sympathetic Cardassian to cure a childhood disease) might have passed muster when dealing with any ordinary doctor or field medic. But when she tries to use it on the ship's Emergency Medical Hologram, who is essentially a walking database of all known medical knowledge in the Federation, it falls apart in a heartbeat.
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.
In an episode of Wallander, the title character avoids a fatal bullet by tripping over a conveniently-placed rug.
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...
In White Christmas, Kang Mi Reu sneaks back into the school unbeknownst to anybody but his fellow students to pull a prank after being expelled. The killer never knew he existed and when he reveals himself and holds the other kids at gunpoint, Kang Mi Reu had since left the school. Though unable to navigate the snow Mi Reu returns and tries to pull a prank on an unsuspecting person, in actuality the killer, by wiring a doorknob to shock him. It disables the killer long enough for them to get the gun away from him and reclaim the school. Only then does he realize that a dangerous serial killer had been threatening everybody.