Adventures of Sonic the Hedgehog: In the episode Grounder the Genius, Grounder installs Robotnik's Genius Chip by accident, and becomes both genre savvy and almost psychic with his intelligence. He employs what is functionally a Xanatos Gambit from his angle wherein he knows what Sonic will do, how to stop him, what his friends will do, and even how to stop them. He even admits to knowing they'd break Sonic free and anticipated that ahead of time. The only reason he fails is because Scratch hears an explosion and concludes Grounder isn't calling him because he thinks Scratch is unhelpful, which somehow causes Scratch to decide to rush up immediately to help to prove Grounder wrong. He then trips on a trap made by Sonic that Grounder had just chucked aside, accidentally knocks Grounder's head off, and Grounder is surprised Scratch would actually show up to help at all. Thanks to this Robotnik gets the Genius Chip, Sonic swaps it for the Stupid Chip, and Robotnik undoes everything Grounder put together (who, himself, has returned to being a moron without the Genius Chip}.
In Aladdin: The Series, Sadira uses a magical sand to rewrite the memories of everyone in the kingdom so she's now the Princess and Jasmine believes herself to be a "street rat" thief. Even the Genie is effected as Sadira prepares to marry Aladdin at last. However, she doesn't realize until the spell is cast that somehow animals are unaffected by it. Thus, Iago, Abu and Rajah are quick to realize what's happening and go to find Jasmine to set things right.
Archer: In the two-part season 3 finale, Pam and Cheryl stow away on the spaceship, causing them to miss their trajectory for Mars and land on the Intrepid. Later, when Pam, Malory and Cheryl are held up on the ship, Pam decides to lock out the bad guys to stall for time until Archer rescues them. her plan is foiled by Cheryl, who opens the door and lets the bad guys in so she can become their Martian Queen.
Often, Archer himself is the spanner to some scheme that would normally work except his combination of skills and bumbling ruin it.
On at least two occasions, Mallory fakes a big bombing threat (once for an exclusive restaurant, the other for a private airship) just to get a free meal/trip via the "investigation." As fate has it, there really is a bomb plot underway each time that would have worked had Mallory's selfishness not gotten the team involved.
Avatar: The Last Airbender: Long Feng's plan to trick Team Avatar into leaving Ba Sing Se would have worked out perfectly if Smellerbee and Longshot hadn't just happened to cross their path and inadvertently reveal that Jet had been brainwashed.
Katara and Sokka's mother, Kya. For background context, the Fire Nation was trying to destroy Southern Waterbending. After nearly 100 years, they had seemingly succeeded, but a new one was born to Chief Hakoda and his wife, Kya. Somehow, they learned of the existence of the Last Waterbender (none other than Katara), and launched an invasion to find and eliminate her. When they did, they questioned Kya as to the identity of the last Waterbender. Wanting to protect her daughter, she lied and claimed she was the Waterbender. She was promptly killed and the Fire Nation retreated, but never realized they got the wrong person. So Katara grew up with an intense hatred of the Fire Nation, and one day lost control of her powers in a tantrum, which led to discovering the Avatar, Aang, in an iceberg. This was the catalyst that eventually led to the Fire Nation's defeat, and none of it would have happened if not for Kya. Her lie saved the world.
Lydia occasionally serves as this in the Beetlejuice cartoon, especially when the title character's assorted enemies are up to no good. One episode in particular, "The Neitherworld's Least Wanted," has a Villain Team-Up plot; the bad guys successfully trick Beetlejuice into physically falling apart, which causes him to lose his powers. What they didn't bank on was him being able to contact Lydia in time for her to come to his rescue and help him pull himself together (literally).
Happens every time Crusher cheats in Blaze and the Monster Machines. Crusher's cheating to stop Blaze from following him would've worked if a) Blaze hadn't turned into something to get past it, and b) Blaze didn't discover a slight downfall in his plan, allowing him to get right past.
Used explicitly and spectacularly in the second C.A.K.E.D. episode of Codename: Kids Next Door. At the episode's climax, Nigel responds to the Delightful Children's five-person-unison speech about having planned every last detail by declaring that they forgot one thing: "MY CRAZY GIRLFRIEND!"
Deedee from Dexter's Laboratory is almost always the one to ruin her little brother's scheming. Dexter turns this into Flaw Exploitation when he learns about his Arch-Enemy Mandark's crush on Deedee, and turns her loose in his lab.
As deranged as some of Zordrak and Urpgor's schemes to steal The Dreamstone are, some of them almost do the job were it not for Sgt Blob and his soldiers' blundering. The odd occasion Urpgor plays an active part in a mission he usually proves to be just as detrimental.
Ed, Edd n Eddy. The Eds' scams might have worked if Ed weren't so stupid to break them.
Eddy's own overzealousness, greed, and big mouth have meant he's been the Spanner to his own scams just as frequently as Ed, if not more. Even Edd will do something to get in the way on some rare occasions. In truth, the friends are in a bit of a Catch 22: one of the Eds is going to be the spanner on any given scam, but none of their scams would even get close to success without them working together in the first place. In the very rare event that none of the Eds screw up, the Kanker Sisters will show up and ruin everything.
Family Guy: Stewie appears on Kids Say the Darndest Things intending to hypnotize the viewers with a mind-control device. This plan gets foiled when Bill Cosby confiscates his headset and mistakes it for a pair of skiing goggles.
The Flying Brains of Futurama intend to collect all the information in the universe and then destroy it, using their telepathic powers to keep anyone from stopping them. But they didn't count on Philip J. Fry, the one man in history too stupid for those powers to affect.
House of Mouse: While Mickey runs the club, Pete owns the building and wants nothing more than to shut it down, but he's bound by their contract, which forbids it as long as the show goes on. In one episode, we get two instances of this: his plot was sabotaging the air conditioning in the middle of the summer, driving the customers away. With no audience, there's no show, but then it turns out that there's still one customer: Hades, who is perfectly comfortable in the heat. Pete spends the episode trying to drive him away, but when he finally succeeds by flooding the place, Ariel shows up instead.
Inspector Gadget is a perfect example, as he often inadvertently helped Penny and Brain solve the cases through his clueless bumbling:
Penny and Brain are trying to prevent a nuclear missile from being launched at Metro City, but it's taking too long and the countdown has started. We then see Gadget wandering through another part of the MAD complex, where he damages some important equipment. This aborts the countdown and gives Penny and Brain the time they need to disable the missile.
Gadget is swallowed by the robotic Stock Ness Monster that Dr. Claw has turned loose in an important lake, and he proceeds to start messing around with its inner workings. This causes the monster to go haywire and start swimming around the lake at random, which allows Penny and Brain to get inside of it and take control themselves.
Gadget is surrounded by MAD agents in a cave, and tries to get away by using his Gadget copter. The copter's rotor blades become stuck in the cavern ceiling and cause Gadget to start spinning out of control. His Gadget arms and legs start flailing around, pummeling the MAD agents and knocking them senseless.
Some MAD agents are about to spray a toxic wood-rot formula over a forest from an airplane. Penny tries to use the woodrot formula to knock a tree onto the runway to keep the plane from taking off, but the spray nozzle clogs and she only sprays enough to weaken the tree. Along comes Gadget flying in the Gadget copter, and he crashes into the weakened tree just as the plane is about to take off, knocking it over and stopping the plane dead in its tracks.
And then there was the time Gadget was in one of Dr. Claw's undersea bases, and opened the seal that kept the place from being flooded...
Gadget is pursuing a MAD agent in the Gadgetmobile, and Dr. Claw arrives in the MadMobile to try and stop him. In trying to catch the MAD agent, Gadget repeatedly activates the wrong gadgets, unknowingly spraying Dr. Claw with laughing gas and then firing a missile at the MadMobile that leaves it trapped in glue and unable to stop Penny from taking control of the MAD agent's vehicle to finally capture the villain.
MAD has taken control of the new supercomputer that the Metro City Police Department uses to coordinate its efforts to fight crime. When Gadget tries to use the computer to contact Chief Quimby, he thinks that the machine is broken and tries to "fix" it. He inadvertently wrecks MAD's connection over the computer, and then alerts the police to what's going on, enabling them to catch all the MAD agents who were committing crimes while the police were distracted.
Gadget is in a Spot the Imposter situation with a MAD spy who's disguised exactly like him. No one can tell which Gadget is real until our hero stands next to Chief Quimby. Gadget's mallet activates by itself and bonks Chief Quimby on the head, and the dazed Chief immediately orders that the other man be arrested, since the one who hit him is obviously the real Gadget.
Zim is often enough his own spanner. He's also a spanner for the whole Irken empire, being the sole reason why Operation Impending Doom 1 failed. The reason the Tallest gave him Earth (which was just off their starmaps and which they didn't know existed) as an assignment was because they wanted to get him out of the way of the real Irken forces so he wouldn't do this.
In "Big Bad Rubber Piggy", Zim's plan to kill Dib in the past seems to work without a hitch, but then Professor Membrane puts his crippled son in a mech suit, giving him the power of ten thousand little boys!! Cue the absurdly powerful present day Dib going on a Roaring Rampage of Revenge throughout Zim's base. Zim is forced to hit the Reset Button just to save his life.
Ron Stoppable. And when the bad guys remember to account for Ron, out pops Rufus.
In one episode Dr. Drakken goes so far as to leave Kim's father out of his gathering of geniuses-to-turn-idiots. When demanded to tell him why, Drakken states its because he realized kidnapping him would just get Kim's attention. He didn't count on his scheme being right next to the ranch run by Kim's uncle and she, Ron, and her father being there on vacation at the same time, though.
"Lighter Than Hare", where Yosemite Sam (hilariously cast as a space alien) thinks that Bugs Bunny would be so foolish as to take on his indestructible tanks and unbeatable robots. All Bugs needs is a simple dynamite stick to easily beat back the challenge.
Various cartoons pitting Bugs Bunny vs. Wile E. Coyote, with the Coyote hoping to use his brains and elaborate scientific contraptions to capture and make mincemeat out of Bugs. Only it is his intended prey that foils the Coyote — and all he usually needs is a dynamite stick or to pull a switch.
"Wet Hare" is built on this trope. Here, Bugs' opponent, Black Jacque Shellacque, thinks he's got Bugs' next moves figured out by building dams of various strengths. All Bugs needs to do is trick Jacque into pulling a tiny rock from near the dam's base, or (after Jacque mistakes a phony shark for a real one(!)) getting the shark to ram another dam, to softening Jacque's inhibitions by distracting him with a single dynamite stick ... before sending a huge raft of explosives crashing into the dam to beat him back. (Bugs eventually gets rid of Jacques once and for all by tricking him into trying to blow up the federally controlled Grand Cooler Dam!)
Coop of Megas XLR does this constantly. Prime examples: he once destroyed a planet-eating monster by firing an EMP-missile-turned-fridge packed with Pop Rocks and Coke, and in another episode he destroys the Glorft mothership by accidentally beaming his slushie onto one of its control boards. Not to mention all of the times he's done it to himself and other good guys.
Milo Murphy's Law: The titular character deals with Murphy's Law on a daily basis, where anything that can go wrong usually does, usually messing with whatever plans his and his friends may have that day. However, when you mix villains into the equation, all the unfortunate events get directed at them. In "Missing Milo", he puts his backpack on a falling tree. After handing out most of the contents of his backpack to his friends, he throws a baseball at King Pistachion. Later, he blows on a woodpecker whistle, and the birds peck away at Pistachion, followed by Diogee taking a tinkle on the mutant pistachio plant, since uric acid is dangerous to that particular breed of pistachios.
In "Slice of Life", Matilda's and Cranky's wedding gets moved ahead a day earlier than planned due to a misprint by Muffins when printing the invitations. While Doctor Hooves is attempting to unwind in a game of bowling, Muffins comes up to him and asks if she can use his flameless fireworks in place of flowers. Hooves agrees, and later on, when it's time for the wedding, Muffins closes the town hall door, accidentally locking Twilight and friends out of the wedding. As soon as Matilda and Cranky kiss, the flameless fireworks go off, and Doctor Hooves discovers that the flameless fireworks he thought were duds needed love to ignite.
King Sombra in "The Crystal Empire" was only thwarted because Spike went along with Twilight against her wishes and helped her get around all the traps that King Sombra had placed to guard the Crystal Heart. It's also Spike who delivers the Crystal Heart to Princess Cadance after Twilight gets caught in the final trap and can't deliver the Heart herself. This is especially notable because, according to the original plans of both the good guys and the bad guys alike, the spanner was never supposed to be there in the first place.
Twilight Sparkle served as this in "A Canterlot Wedding", as Queen Chrysalis didn't count on Twilight having such a close bond with the real Cadance. Her cold, snobbish behavior towards Twilight caused her to become suspicious, and though Twilight is unable to give her friends solid evidence that something's wrong, Twilight's actions and Chrysalis' attempt to get rid of her allow the real Cadance to escape imprisonment, reunite with Shining Armor, and use their love to defeat the Changeling army.
Starlight Glimmer becomes this in "To Where and Back Again." While Chrysalis' forces manage to capture the Mane Six and the princesses, Starlight was considered barely worth acknowledging. Thanks to this, Starlight is able to gather a group to infiltrate the changeling hive and get Thorax to destroy the Anti-Magic darkstone throne, not only freeing everyone but also leading to the entire changeling hive turning against Chrysalis.
While Candace Flynn isn't evil, her attempts to show her mom what Phineas and Ferb have done would be more successful if Dr. Doofenschmirtz's crazy inventions didn't inexplicably cause their projects to vanish.
Pinky is the downfall of pretty much all of Brain's schemes.
Inverted in one episode, where Pinky convinces all of the world's leaders to hand over control of the world to Brain on a silver platter, only to have Brain himself torpedo the plan.
There are several occasions where it's Brain's own oversights that doom his plan (the Jeopardy episode where he blew the last question due to it being a pop culture one, as an example. And Pinky is completely obsessed with the show the Final Jeopardy question is based on. If only Brain listened to him...
There's also the episode where Brain builds a machine to calculate exactly what common factor keeps causing his plans to go wrong. Sure enough, the catalyst is not Pinky, but Brain himself. Which raises the question of how Brain could fail to be a common factor in all of his own schemes.
Lampshaded in a Kids' WB! promo for the show, in which Pinky confesses:
"I'm not really that stupid. I purposely sabotage Brain's plans because if he ever succeeded, the show would be over, wouldn't it?"
The Powerpuff Girls: "Three Girls And A Monster" has Blossom and Buttercup going through white heat trying to bring down a giant lizard monster, arguing with each other over which works best—brains or brawn. Sweet little Bubbles gets rid of the beast—by politely asking it to leave.
In one episode of Rocko's Modern Life, Ed Bighead is recruited to play golf with Conglom-O's boss and told to play poorly. Ed agrees and happily plays poorly... with the help of cannon-launched pianos. However, Heffer ended up working at the golf course they were playing at and made it his duty to get Ed to win. Thankfully, the ensuing craziness just made the boss request Ed play with him again next time.
Bullwinkle, of Rocky and Bullwinkle, is an idiot whose sheer stupidity endlessly frustrates villains Boris and Natasha.
Natasha: I thought you said this plan was foolproof! Boris: Foolproof, yes. Idiotproof, no!
In the Rugrats episode "The Bank Trick", Tommy and Chuckie mistake an ATM machine at the bank for an M&M machine and think the bank is actually a candy store. They don't find any, but they do inadvertently trip a security alarm and expose two bank robbers posing as federal bank inspectors.
Many a Scooby-Doo villain has seen their counterfeiting, diamond smuggling, theft, fraud, or other criminal scheme foiled by "those meddling kids!" Since the Scooby Gang usually rolls into town without any knowledge of a ghost or a monster running around beforehand, one assumes that some or all of those schemes would have worked beautifully if the Gang hadn't unwittingly injected themselves into the situation.
This is Loo-Kee's role in both of his Day In The Lime Light episodes on She-Ra: Princess of Power, though it's most obvious in "Loo-Kee Lends a Hand." The Horde Timestopper had successfully frozen the Rebellion, including Adora/She-Ra, but since they didn't know about Loo-Kee, who's powers revolve around hiding and escaping, he was able to no-sell the attack and retrieve help from Eternia.
The South Park cast finally decide to ignore Cartman after he eats the skin off all their KFC chicken. Cartman thinks he died as no one communicates with him and Butters is the only one who can see him. A psychic explains that he is stuck on Earth to deal with a crisis. After learning of a hostage situation, Cartman believes he is the only one who can help. Believing he can't be harmed, he freely walks into the Red Cross Center and pretends to be a ghost while moving items around. The robbers don't know what to do and the police move in once Butters frees the hostages. The two are credited for saving the day "armed only with the weapon of confusion".
Miss Lion, Aunt May's dog, in "Seven Little Superheroes". By tagging along with Peter Parker-as-Spider-Man on a trip to a mysterious island, she is able to foil The Chameleon's scheme to separately kill off Spidey, Iceman, Firestar, Captain America, Doctor Strange, Namor the Sub-Mariner and Shanna the She-Devil, particularly when she can sniff out evil drones disguised as various X-Men.
The episode "Spidey Goes Hollywood" had Mysterio's scheme to terminate the Spider-Friends unintentionally thwarted by the Hulk.
In SpongeBob SquarePants, we have SpongeBob's best friend Patrick Star, who has a tendency to make any bad thing worse. In fact it was so bad that it was actually teased and subverted in "House Sittin' For Sandy", where SpongeBob, who was taking care of Sandy's treedome, told him not to do anything. Then, in a surprising twist, it's Spongebob who accidentally causes a Disaster Dominoes effect that destroys Sandy's treedome.
For all of his expertly-planned machinations, Palpatine/Sidious and his apprentice Dooku did not count on the latter's disciple, Asajj Ventress, surviving their attempt to betray and kill her. This leads to Ventress seeking out Mother Talzin for aid against Dooku.
Ironically, Palpatine himself is a Spanner for Maul's own ambitions. Just when he has gained his own apprentice (his brother Savage Opress), conquered Mandalore, and finally gotten revenge on his archnemesis Obi-Wan Kenobi, who should show up to spoil the triumphant moment but his former master?
Star Wars Rebels: In "Warhead", the Empire sends out infiltrator droids to scout planets where the rebel base could potentially be located. One of them lands on Atollon, where the rebel base is located and is shortly attacked by krykna, the notoriously resilient local wildlife, and damaged before it can report its location. The damage proves instrumental to the rebels being able to postpone the eventual discovery of their base.
"Cry for Help": Peridot has repaired the Communication Hub from "Coach Steven", and Garnet and Pearl fuse into Sardonyx to take it apart. This happens several times, as the hub is repaired each night. It's eventually discovered by Steven and Amythest, on a stake-out to catch Peridot in the act of repairing the device, that Pearl was repairing the hub in order to constantly feel the power that comes from being fused with Garnet, resulting in a short story arc about Garnet dealing with her feelings of betrayal and the duo's reconciliation. Had the younger members of the team not snuck out to investigate the hub, it's likely that Pearl's activities would have caused a serious delay in finding Peridot, and Pearl would have continued to be insecure about her abilities and keep using Garnet. Also, had Garnet found out any later, it's possible that she would never have forgiven Pearl for misusing fusion, which she takes very seriously.
Peridot herself is this. She's a minor cog in the Gem Empire's machine, but she thwarted Yellow Diamond's plan to destroy Earth by informing the Crystal Gems about the Cluster (which would have caused an Earth-Shattering Kaboom) and then helping them to stop it, first because she was stranded on Earth, and later because she had grown to care about the planet.
The Crystal Gems themselves were spanners in this scenario. Yellow Diamond had thought that the Crystal Gems had been taken care of by Japser, and even if they hadn't, they lacked the knowledge that the Cluster even existed, so they'd be destroyed regardless.
Ruby, and by extension Garnet, is a spanner in "The Answer". Way back at the beginning of the Crystal Gem Rebellion, she was assigned along with two other Rubies to bodyguard a Sapphire. The Sapphire was supposed to be poofed in an upcoming rebel attack, after which the rebels would be captured. However, Ruby survived Rose Quartz poofing the other rubies and rushed in to save Sapphire from Pearl, which resulted in them fusing into Garnet by accident. The distraction gave Rose and Pearl the opportunity they needed to get away. Actually, this incident was what lead to the creation of the Crystal Gems in the first place, as revealed in "Now We're Only Falling Apart". The Crystal Gems were originally a bogeyman created by Pink Diamond as an excuse to not complete her colony, but Garnet's formation made her realize that there were other gems suffering under Homeworld's oppressive caste system and made her decide to make Earth a haven for outcast gems, which formed the Crystal Gems for real.
"The Trial": Steven is on trial in a Kangaroo Court set up by the Diamonds. The Defense Zircon becomes a spanner by spotting several inconsistencies with the story of Rose Quartz shattering Pink Diamond. The Defense Zircon concludes that Pink Diamond was killed by someone close to her, someone who could get past Pink's entourage with no interference, someone who'd have the authority to have the entire situation covered up. Someone... like another Diamond. The Defense Zircon gets poofed immediately, but the arguing between Yellow Diamond and Blue Diamond that results creates just enough confusion to allow Steven to escape.
In "Exodus Part 1", Agent Bishop and his commandos, operating separately of the Turtles, attack Shredder's spaceship, forcing him to launch early, leaving the majority of his forces on Earth. While it doesn't destroy Shredder's plan completely, it does make things really difficult for him.
Oroku Saki: Bishop here!? This changes everything!
Yukio betrayed the Utroms to Ch'rell, prompting Hamato Yoshi and Splinter to move to America. In a way, he's responsible for creating the Ninja Turtles that will be Ch'rell's downfall.
Mirta from Winx Club manages to derail the Trix's scheme to trick Bloom into thinking she was descended from the Ancient Witches. Later, when she's a pumpkin, she spots the Trix's nightmare monster and warns the Winx, allowing them to prepare for it.
Cedric in the second season of W.I.T.C.H.. Will had a Batman Gambit set up so that Prince Phobos could gain the power Nerissa stole and, when he would attempt to wander into Kandrakar, he would lose it and they would get it back. But nobody had expected Cedric to get tired of Phobos' crap and eat him.