History SpannerInTheWorks / WesternAnimation

** Entrapta started changing the plot from the moment she was accidentaly left in the Fright Zone. Her explanation about the true function of the runes encourages Catra to get the Black Garnet and put an end on Shadow Weaver's chronical abuse. Entrapta's hacking of the Black Garnet causes an environmental disaster, but it also brings back Mermista and Perfuma to Alliance, and Frosta also joins them. Besides, that causes Shadow Weaver's downfall , so she runs to Bright Moon where she eventually teaches magic to Glimmer.I f Catra hadn't been patient enough to hear Entrapta's explanation, Glimmer wouldn't have awaken her true potential for magic, in spite of having a magic being for a mother and her aunt being the head of Mystacor (talk about overprotectiveness). By other turn, Shadow Weaver gives Glimmer bad advices that cause terrible consequences.
** Shadow Weaver's escape is also a spanner because it makes Catra fall in disgrace, too, worsening her psychological issues. Once again, Entrapta interferes when she talks Hordak into sending Catra to Crimson Waste instead of Beast Island. Catra takes over a local gang, captures Adora and uses her sword to open the portal and destroy the reality, [[spoiler:forcing Angella to commit a HeroicSacrifice and Glimmer being crowned as the new queen]]. Worse, [[UngratefulBastard Catra sends Entrapta to Beast Island]], so the scientist wouldn't warn Hordak about how dangerous the portal was, and tells Hordak that Entrapta betrayed him. That breaks Hordak's spirit, making him vulnerable enough to be beaten by Catra, who takes over the Fright Zone and eventually causes the fall of the Etherian Horde. Not to mention that Scorpia's remorse for [[BetrayalByInaction not standing up for Entrapta]] causes her to defect and join the Alliance. If Hordak had sent Catra to Beast Island instead - or if Catra decided to have a new life on Crimson Waste, as Scorpia suggested -, probably Entrapta and Hordak would keep pretending to build the portal for a long time and maybe Hordak would eventually give up on returning to Horde Prime. Since Hordak was conquering Etheria just to impress Horde Prime, maybe he would eventually give up on that, too.
** By other turn, if Catra hadn't sent Entrapta to Beast Island, nobody would learn that [[spoiler: Micah was still alive]].

* ''GIR'' the CloudCuckooLander robot from ''WesternAnimation/InvaderZim''.

* ''WesternAnimation/TheAvengersEarthsMightiestHeroes'': In "Masters of Evil," Baron Zemo is thwarted because he didn't realize Comicbook/BlackPanther and Comicbook/{{Hawkeye}} had joined the Avengers, and thus failed to take their presence into account when coming up with his plan to destroy the team.

* From the ''WesternAnimation/JusticeLeague'' episode "A Better World": An evil (or rather, corrupted KnightTemplar) alternate universe Justice League has captured the "real" League. Their Batman designed inescapable prisons for each Leaguer, [[CrazyPrepared because he's]] Franchise/{{Batman}}. The Flash manages to escape by [[FakingTheDead speeding up his heartbeat so it looked like he flatlined]], and when the other Batman unlocked the cell, Flash beats him up at super speed. {{Lampshade|Hanging}}d by our Batman:
-->'''Batman''': He anticipated everything I would have thought of. But who could anticipate you?
** Indeed, Batman is surprised to know Flash could have pulled that trick and Flash admits he's never actually tried it before so it was no wonder the Justice Lord Batman couldn't have seen it coming.

* ''WesternAnimation/SpiderManAndHisAmazingFriends''
** 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.
** "Spidey Goes Hollywood" has Mysterio's scheme to terminate the Spider-Friends unintentionally thwarted by the Hulk.

* ''WesternAnimation/SupermanTheAnimatedSeries'': In "Knight Time", Roxy Rocket, the villain from the ''WesternAnimation/BatmanTheAnimatedSeries'' episode "The Ultimate Thrill", inadvertently ruins Brainiac's plan by informing Superman that Batman went missing. She didn't even know Brainiac was involved.

* ''WesternAnimation/{{Rugrats}}'':
** In "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.
** In "Ice Cream Mountain", Stu and Drew Pickles are competing at miniature golf, where the course's final hole, "Ice Cream Mountain", allows a free game to golfers who make a hole-in-one. It's apparently a one-in-three shot, as the structure they hit the balls into has three outlets, one of which leads straight to the hole, [[UnwinnableByDesign but the course owner blocked up that hole]]. That is, until the babies, who were supposed to be taken out for ice cream, find it, believing it's a literal ice cream mountain, and Angelica ends up blocking the other two holes instead, leading to Stu and Drew, along with other golfers, making holes-in-ones and getting free games, much to the owner's horror.

** In "Ice Cream Mountain", Stu and Drew Pickles are competing at miniature golf, where the course's final hole, "Ice Cream Mountain", allows a free game to golfers who make a hole-in-one. [[UnwinnableByDesign However, the course owner has fixed it so that nobody would get it]]. That is, until the babies, who were supposed to be taken out for ice cream, find it, believing it's a literal ice cream mountain, and inadvertently unrig the last hole, leading to Stu and Drew, along with other golfers, making holes-in-ones and getting free games, much to the horror of the owner.

* In ''WesternAnimation/{{Amphibia}}'' season two finale "True Colors" Grime and Sasha's Toad Rebellion [[spoiler:turns out to be this for Andrias's true plans for the Calamity Box, having gotten the drop of him and taken over Newtopia just before Anne could give him the box. Upon discovering proof of Andrias's actual nature, the pair even lampshade how their coup ended up being a ''good'' thing. Unfortunately, Anne and the Plantars, not knowing the truth, thwarted their rebellion and put Andrias's plans back on course, leading to the third season's events]].

* ''WesternAnimation/BeavisAndButtHead'', of all people, pull this off in ''WesternAnimation/BeavisAndButtHeadDoAmerica.'' In the hope of "scoring", the dimwitted duo travel to Las Vegas and foil a criminal terrorist plot without realizing it.

* Deedee from ''WesternAnimation/DextersLaboratory'' is almost always the one to ruin her little brother's scheming. Dexter turns this into FlawExploitation when he learns about his ArchEnemy Mandark's crush on Deedee, and turns her loose in ''his'' lab.

** 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.

* In WesternAnimation/TheSimpsons episode ''[[Recap/TheSimpsonsS4E10LisasFirstWord Lisa's First Word]]'', Krusty and his Fast food restaurant, Krusty Burger, holds a promotion during the 1984 Olympic games where they print game cards for different events and if the US won that event, customers can redeem the card for a free Krusty Burger. Krusty and his accomplices think they will get very rich since the game cards have been rigged to favor Communist countries that were good on those events. Unfortunately, they just learn the Soviet Union and other Communist countries have boycotted the Olympics, meaning those Communist-favored games will now be won by the US. [[https://laist.com/news/food/mcdonalds-olympics Inspired by a true story]] (except for the rigging of the cards).

* ''WesternAnimation/TheOwlHouse'': King manages to be this for the season 2 finale. Belos's plan for the Day of Unity is going off without a hitch, and he even managed to plan for the good guys' attempt to use Eda and the Owl Curse as this. He even [[spoiler: betrays the Collector, refusing to use the Titan blood he has to free the being]]. What Belos didn't count on was the Hexside squad bringing King, an actual Titan, to his location, wherein King seizes upon an opportunity to stop Belos by [[spoiler: freeing the Collector and stopping the draining spell.]]

*** And this gives Rutherford, who has been obsessing over the ''Texas''-class, enough time to discover something that supersedes Freeman's case against the ''Aledo'': ''he'' coded the AI used in the class, and Buenamigo erased his memories to cover up his involvement. This means that a Starfleet officer erased a cadet's memory of him, which he'd only do if he had something to hide. More urgently, because he unwittingly forked that AI when he created Badgey, he's aware of a problem with the AI that makes the ''Texas''-class ships entirely unfit for duty... namely, that there's an [[AIIsACrapshoot emotional processing glitch]] that predisposes any AI with that specific code to go AxCrazy and [[TurnedAgainstTheirMasters become hostile towards its "father"]]. Buenamigo is forced to admit that he set the ''Cerritos'' up for failure--which, given that he got the ''Cerritos'' attacked by the Breen, is reckless endangerment ''at best''--to get his pet project off the ground, and his attempt to avoid legal repercussions for what he did [[KarmicDeath ends up getting him killed]].

*** And this gives Rutherford, who has been obsessing over the ''Texas''-class, enough time to discover something that supersedes Freeman's case against the ''Aledo'': ''he'' coded the AI used in the class, and Buenamigo erased his memories to cover up his involvement. This means that a Starfleet officer erased a cadet's memory of him, which he'd only do if he had something to hide. More urgently, because he unwittingly forked that AI when he created Badgey, he's aware of a problem with the AI that makes the ''Texas''-class ships entirely unfit for duty... namely, that there's an emotional processing glitch that predisposes anything with that AI code to go AxCrazy and [[TurnedAgainstTheirMasters become hostile towards its "father"]]. Buenamigo is forced to admit that he set the ''Cerritos'' up for failure--which, given that he got the ''Cerritos'' attacked by the Breen, is reckless endangerment ''at best''--to get his pet project off the ground, and his attempt to avoid legal repercussions for what he did [[TurnedAgainstTheirMasters ends up getting him killed]].