The entirety of The Grim Adventures ofthe Kids Next Door was just one big Idiot Plot that involved all kinds of KND and Billy & Mandy characters falling for some of the most pathetic Paper-Thin Disguises in fictional history. We can expect this thing from those minor Billy & Mandy characters but every single KND moon base operative too? The only ones without an Idiot Ball attached were Mandy, Grim, Numbuh 5, The DCFDTL, and (most of the time) Numbuh 1. Billy, on the other hand, is an Idiot Ball, so he doesn't count.
As mentioned above, this is standard fare for The Grim Adventures of Billy & Mandy, to the point where few episodes begin without being catalysed by Billy AKA Idiot Ball incarnate and Grim and Mandy stupidly giving in to his demands. Sure, Grim is their best friend/slave forever, but one expects better from the relatively Genre Savvy Mandy (though it could be because she enjoys watching Billy in pain).
You'd think after over 50 years, Scooby, Shaggy, and the rest of the gang would have the common sense not to run from every monster they see after so many frauds. Might be justified though, as thefew times they do think it's a hoax, it turns out the monsters are real.
And even if they know it's a hoax, often the monster is dangerous in their own right, but virtually every monster could get taken down if faced by more than a single unarmed cop so then the question is why they never call authorities against say a lone ghost miner.
Happens more than it should to poor Eddie Spenser, Jr. on Filmation's Ghostbusters. It doesn't help that he gets a few episodes where he's quite capable of busting ghosts along with the best of 'em.
The various Super Mario Bros. cartoons were often driven by the characters being suddenly weakened to the point where they walk into or can't escape traps that they would have easily dealt with before. Other ridiculous examples include them just standing and watching as Harmless Villain King Koopa tosses Chain Chomps at them, which latch onto their ankles like makeshift manacles and leave them easy to capture.
One Chaotic episode involves Kaz's science teacher receiving scans from the UnderWorlders in exchange for the technology to travel between dimensions. Did he not stop to consider that if Chaotic was invaded, all his scans would be worthless? (It was All Just a Dream though.)
An even more blatant one is the episode where Kaz is in danger of being able to return to Chaotic because his mother will take his scanner away, all because Kaz keeps insisting, time and again, to everybody in his vicinity that Chaotic and Perim are real and he even does his school assignments based on his adventures there, this despite having absolutely no way to prove (except for a Contrived Coincidence at the end of the episode) that these places actually exist.
In the episode "Where Is Thy Sting?," first when Bumblebee and Wasp switched places with none of the other Autobots noticing that Bumblebee (really Wasp) has purple eyes as oppose to his normal blue ones. This was visible even with Bumblebee's battle mask up. Then, Optimus and Ratchet get into a battle with Jetfire and Jetstorm of the Elite Guard which could have been avoided.
The Transformers had many of these, most famously in "A Prime Problem" where Megaton constructs an exact replica of Optimus Prime that he controls via remote, only to have the real Optimus show up, and Teletraan 1 inexplicably is unable to tell them apart when scanning them. Instead of, you know, asking the Primes questions only the real Prime would know or not noticing that the fake Prime mixes up Ironhide and Bumblebee (which given the fact that Megatron has been fighting these guys for millions of years and should freaking know them by now, is another level of stupidity), their solution? Have them race to see who's the fastest. Became a bit of Memetic Mutation in the fandom.
And of course numerous episodes where the Decepticons are shown the ability to create incredible machines that do unworldly things, yet half of the season 1 and 2 episodes revolve around them trying to steal rather mundane human inventions that they, could, you know, probably create themselves without drawing attention.
Nearly every single episode of the original ThunderCats relies on the titular characters being as thick as possible. Lion-O, with a firm grip on the Idiot Ball, is the worst offender, though since he's a kid in an adult body it's somewhat excusable. Occasional episodes, however, will have him pass it off to the others, usually Tygra who gaily runs with it straight into trouble.
On Phineas and Ferb, the incredibly awesome Fireside Girls are trying to raise money to protect an endangered critter. They ask P&F's help because their attempts at making money, like running a lemonade stand, ended in disaster. Unlike other instances of sudden ability lack on the show, it is not presented as ironic or even pointed out that these girls have been a race-pit crew, mission control on a space flight, and rebuilt a time machine despite faulty instructions. Yet they cannot run a lemonade stand. However, given that this is brought up 35 seconds into the episode, sets up a different plot and involves one of the Fireside Girls inexplicably covered in bandages, it's possible that this just counts more as Rule of Funny.
The Idiot Plots don't detract from the show since they're a large part of what drives it along, but they're there nonetheless (and, as stated, are a HUGE part of what drives it along). The only reason nobody discovers Timmy Turner's fairies is because of the stupidity of everyone else. Even when they're disguised as ordinary objects, with obvious faces, and they speak while in this state, in front of large crowds. Timmy's parents (and even random people on the street at times) will, instead of questioning Timmy's various talking possessions, simply remark that "That [object that clearly should not be speaking but did] is right!"
Timmy's Mom: Those gleeful goldfish are right to cheer!
Even before the obligatory reset button at the end of each episode, people rarely notice or care about the huge, unexplained changes in Timmy's behavior/appearance/life or the city of Dimmsdale as a whole, even if they're personally affected.
Even more obvious is how most of Timmy's bad wishes that fuel episode plots could be easily resolved with one counter-wish. (Usually handwaved with "Magic can't effect [antagonist]!" or one of Da Rules blocking the counter-wish.)
Most of Timmy's wishes border on Too Dumb to Live territory, but Wanda and Cosmo generally say "okay, your funeral" instead of explaining why his wish is insane; Wanda has repeatedly gone on vacations or personal business alone when she knows that leaving Timmy and Cosmo on their own can and does result in The End of the World as We Know It.
One of the worst specific examples is "The Grass is Greener". The entire circus plot would literally have not happened had Timmy not overheard his dad talking about him, or more correctly would have not happened had his dad decided to phrase his opinion in literally any way that wasn't basically "Timmy is a worthless little prick who should have been a girl and then I would be much much happier... [Timmy walks out of earshot, sad, to go make preparations to run away] ...is the exact opposite of how I really feel!"
Two episodes had Timmy babysitting Poof. Despite the fact that Poof is immortal, so he doesn't need protection.
As awesome it was, it was not immune to Idiot Plots from time to time. One notable example is the Justice League episode "War World". Though closely adapted from a popular DC Comics storyline from the early 80s, "War World" is easily the weakest episode of the first season of Justice League, in large part because of its flaming idiot plot. This World's Finest review says it best:
"Ebert defines an "idiot plot" as a plot that could be solved in 5 minutes if the characters did not act like idiots. We get this in the very first scene, when Superman, J'onn and Hawkgirl are trying to detonate an asteroid. Something goes wrong with the explosion and Supes and J'onn are left stranded in deep space. We never get a full explanation for what exactly went wrong and who was at fault, a sure sign that the writer, having made one of the heroes an idiot, is trying to avoid pinning the blame on anyone. But if this person had not screwed up, we wouldn't even have a story. Nor is this the only occasion when someone acts like an idiot: Why are GL and Hawkgirl gassed in part two? Because they're arguing like idiots. In the fights on War World, why doesn't Superman just fly away from Draaga? Because he's an idiot."
And then there's the Justice League Unlimited episode "Clash", which relies entirely on Superman acting like an idiotic Jerkass for the story to work. Heck, even Batman points out Supes is acting rashly and out of character and Superman ignores him.
The JL episode "A Better World" only works because when JLord!Martian Manhunter comes to trick the Justice League into coming to his world, everyone takes him at his word, and no one even thinks about staying behind to mind the home fires, provide backup, work on the problem from a different angle, nor sending just an advance crew to verify the story in question. How many shapeshifting, mind bending, illusion-producing villains have each and every member of the JL fought? You would think at this point they would know better than just running off after someone because they tell a good story.
The Batman Beyond episode "The Call" only works because apparently, the Justice League no longer gives physicals anymore, allowing people to walk around for years under mind control, or infested by parasites. How many parasitic, mind-raping, body-snatching villains have each and every member of the JL fought? You would think at this point they would know better...
The Batman: The Animated Series episode "I've Got Batman in my Basement" is a very "embarrassing" example. As Batman ends up unconscious and a bunch of kids hide him in their basement. While Penguin is sneaking around the house trying to get back a treasure that Batman confiscated from him and is in the house, cue Penguin and his mooks acting completely incompetent as they get totally thrashed by the kids reenacting Home Alone. This is especially sad as it's Penguin's debut episode. Even the show's creators don't like talking about it.
Superman: The Animated Series had an episode called "Blast from the Past, part 1" with Mala, a Kryptonian Phantom Zone prisoner, being released because her sentence was rather light. Superman wanted to see if she could help him protect Metropolis and at some point in the episode, Superman loses all sense of compassion towards a person who had clearly been isolated in an Infernal Prison for years and constantly held the threat of being sent back over her head. The Idiot part comes in when you watch the episode and notice that Mala is genuinely trying to be Superwoman and the only reason she descends into villainy is because of the OOC jerkassery of everyone else in the episode.
The entirety of "The Old Man & the C Student" relies on Principal Skinner thinking it's a great idea for Bart to perform an insulting comedy routine aimed at the visiting Olympics committee. When called out on it by Superintendent Chalmers, he admits it was something that sounded a lot funnier in rehearsals.
Even stupider is that the committee had already approved that they passed, and were just about to sign the papers before Skinner insisted that Bart go up.
"Bender's Big Score" had the entire population of the Earth falling for spam ads and obvious Internet scams. Apparently spam filters have become Lost Technology.
Bender even has a virus scanner which catches the spam, but looks at it anyway.
Then again, most characters hold a deliberate Idiot Ball.
Leela (checking e-mail): Spam, spam, junk. (GASP) The very last pygmy rhino is going extinct?! Unless it gets my credit card number?!
"Bender's Game" features an idiot plot set into motion and resolved by Professor Farnsworth in one of his "less than worthy of being a professor" moments. Because fuel prices are higher than they were before, he reasons that the world would be better off if he and his crew went on a difficult quest to destroy the source of that fuel and then replace it with something almost the same... but infinitely inferior, effectively crippling society as they know it. This plot is made even stupider when another character, Bender, becomes so insane that he cannot distinguish fantasy from reality. He eats a shitload of radioactive waste that transports all of the characters into his dream, wherein they still try to accomplish the professor/wizard's idiot plot.
"The Day the Earth Stood Stupid" is an intentional idiot plot, with floating alien brains draining the intelligence of every person on Earth. The only one who's immune to it is Fry, due to lacking a delta brainwave...which means he's already The Ditz to begin with, and ends up only marginally smarter than everyone else.
The Weekenders: Usually a good show, but "Tish's Hair" hinges on Lil Tish (the smart chick of the clique, mind you!) reading a sign that's supposed to say "Your hair is the pits" as "Your hair is it" and not notice that it's been obscured by a tree, and hearing Carver's sister's comment "That's the worst I've ever seen" and thinking it's referring to Carver's shirt instead of her 'do.
The Adventures of Ichabod and Mr. Toad had Toad and friends breaking into Toad Hall to get the deed from Winkie and the Weasels to verify Toad's story. They don't seem to realize that the very presence of the Weasels in Toad Hall would be enough for the police to suspect Toad's story was true and search the place for the deed. Get a constable to see that Toad Hall is "ablaze with lights!" and see whatever Badger saw.
In Teenage Mutant Ninja Turtles (1987), the episode "Zach and the Alien Invaders" is easily the biggest offender of this in the entire series, considering that the whole premise is about Zach convincing the Turtles that he's seen aliens, and robots, and other dangerous things that ended up being harmless, and getting in trouble for it. Because it's not like the Turtles have ever fought aliens or robots before, except you know like every episode where Krang and Shredder attack the Turtles with the Foot Soldier robots which happens on a regular basis! Later on Zach does see aliens that are trying to take over the world, and helps the Turtles stop them, but despite him being right about the attempted alien invasion, the episode ends with a horriblyBroken Aesop of "That's okay, Who listens to kids anyway?", which is why despite featuring the one and only apperances of Wingnut and Screwloose, this is easily still regarded as one of the absolute worst episodes in the entire series.
The episode "Leonardo is Missing" also falls under this, which is lampshaded by Splinter at the end, but unlike the first example, this episode is actually still fun to watch.
Splinter: Next time you leave Leonardo, make sure to leave a note.
"Mikey Get Shellacne" was the Flanderization for Mikey's stupidity. Not only did Mikey think putting more mutagen on him would make him a "super mutant" (despite Donnie actually being Genre Savvy and writing "Mikey do not eat me" on the jar of mutagen), he also turned Dogpound, who was an already sizable threat for the Turtles, into a stronger, faster, more powerful form. Coincidentally enough, Mikey gets his hands on the Smart Ball minutes later after this incident (which is a bit of an Ass Pull considering what this episode has shown).
In the episode "Your Baloo's in the Mail", the entire plot could have been resolved in the first scene if Rebecca had just trusted Baloo and told him that she had a winning lottery ticket that needed to be delivered first class to the lottery office the next morning.
Or she could have done it herself. She just won the frickin' lottery, nothing short of major surgery couldn't be understandably postponed.
In the episode "Save the Tiger", Baloo saves Shere Khan's life in the first scene and Khan wants to repay his debt to Baloo by granting him any request he wants. Considering the fact that Khan said that his company is worth several billion dollars, Baloo could have simply asked for a couple million dollars in cash, bought the Sea Duck back from Rebecca, and lived a comfortable life like he always wanted to. Instead, Baloo constantly pesters Khan with tons of small requests and tries his patience which basically leaves Baloo with nothing in the end.
Invader Zim's whole series is based on this—the story could be resolved instantly if any human except for Dib noticed that Zim was obviously an alien. Their sheer stupidity is part of the humor, though. Even Zim and Dib themselves hold the Idiot Ball a lot, so a lot of specific episodes fall under this trope.
This was the subject of many Aqua Teen Hunger Force episodes. Master Shake was especially prone to these moments. An example is "Broodwich", where Shake could have saved himself a whole episode of trouble if he left the Broodwich alone.
"Joyride". To put it simply, villain Avalanche joins the X-Men to get into Kitty's pants. Scott doesn't trust him. Meanwhile, the New Recruits start taking the X-Vehicles out for joyrides. When it becomes obvious someone is taking the vehicles out for a joyride they start to suspect Avalanche. Two ways this could have been avoided: Lance could have told them it wasn't him and that he saw the New Recruits sneaking around, or Xavier could have checked their minds/Logan could have used his advanced senses to smell the cars and detect who had been using them. In the end Avalanche takes Kitty with him when he realizes they're taking the X-Jet, thanks to their presence, Jubilee accidentally fries the control panel, and they're nearly blown up by the US military. Avalanche then quits because life at the mansion is too hard.
Also, with all of the security at the mansion, things were really flimsy enough that a group of kids who couldn't properly beat a training simulation could crack security and sneak off in the various cars and jets? Surely after it became apparent that someone was using the vehicles, the team would think to at least put a better lock on the garage.
Also, the entire Romance Arc for Lance and Kitty, not to mention that very plot, could have been avoided (along with a lot of heartbreak for both) if she had realized that he was still villainous, since the event that made her think he was nice was because he saved her from a falling statue, a falling statue that HE CAUSED.
Except Lance didn't mean for the statue to fall, and as soon as he realized what was happening, he went to save her. It's also hard to peg the guy as completely villainous when he does things like rescue a crippled old lady from a subway accident when everyone else would leave her to die, try to warn Kitty away from an area where a battle was going to start, and save her life in the Grand Finale even though she wasn't on speaking terms with him before that adventure started.
The entire episode "The Super Speedy Cider Squeezy 6000". The presented "happy" ending of the episode was not anywhere near the best solution, but the entire plot could have been avoided in numerous ways, such as Flim and Flam offering a reasonable business offer, Granny not accepting their bet, Flim and Flam not allowing the Apple family to bring in additional ponies to help, the Apples not forking over control of the whole farm as a result of the bet (it wasn't part of the stakes), or the Apple Family hiring additional help from the get-go rather than attempting to do all the work themselves. In short, if anyone in the cast had even a modicum of business sense, there wouldn't have been any problems.
Even worse is their return episode, where they once again are scamming (despite being called out as frauds and run out of town before) and everyone (not counting the other mane 5) falls for it, except for Applejack. Worst of all Granny Smith herself falls for it the worst, as next to Applejack she should have been the first to call them out on their scams.
Adding to that is that Princess Celestia had never even heard of parasprites. Despite being the freaking goddess who ruled the world for eons. Seeing the massive destruction they can cause (destroying entire towns in a matter of hours) you'd think one of her subjects would have bothered to bring them up at some point.
The season 2 premiere: A lot of this could have been avoided if Princess Celestia had gone with them rather than letting them stupidly wander in on their own. True she can't use the Elements of Harmony, but she already has intimate knowledge of how Discord works since she's faced him before.
To be fair, Celestia's presence might have forced Discord to step up his game, too. He is, after all, a Reality Warper who eclipses her power.
The season 2 finale. Sure, the Mane Six don't know Cadence and assume she's just being a Bridezilla, and Shining Armor is brainwashed, but you'd think that at least Celestia would have picked up on the fact that something is seriously wrong about her freaking niece. Even Twilight Sparkle, Only Sane Mare of the episode, has a brush with the Idiot Ball when she barges into a room and loudly announces Cadance to be evil instead of trying to be subtle about it. On the villainous side, we have such gems as openly admitting you're an imposter after only moments ago fully convincing the only one suspicious of you, imprisoning Twilight Sparkle in almost the same room as the real Cadance, leaving the whole thing guarded by unarmed bridesmaids, and clashing with Princess Celestia (though she wins the fight, it's made amply clear she was not expecting to). Also, even though it's daytime and all, you'd still expect Princess Luna to wake up and help when the city is being invaded...
The season 3 episode "Games Ponies Play". The entire plot revolves around the main cast not realizing that the pony they picked up from the train station wasn't the games inspector, something that could have been completely averted if any of them would have referred to her by name or in any way attempted to confirm who she was.
"The Nasty Patty" episode of Sponge Bob Square Pants where SpongeBob and Mr. Krabs thought they killed the health inspector, when all they did was knock him out. When the police arrive and they confess, the health inspector walks out of the freezer, dazed, leading SpongeBob, Krabs, and even the police to believe he is a zombie! Fortunately, he was not. It was all intentional, however, and the narrator even lampshades it at the very end.
Narrator: Yes, they are all idiots aren't they?
Ben 10: Destroy All Aliens arguably has this bad. The Omnitrix is clearly malfunctioning, but Ben does not tell Tetrax nor anyone else that could deal with it until the halfway point of the film. It doesn't help that, because of these errors in communication, Azmuth, who was visiting to fix the Omnitrix's malfunction, ends up being turned into an angry, incredibly dangerous To'kustar, causing his father to assume that Azmuth died, causing him to attack Ben, whose watch absorbed the To'kustar. This, again, can be traced back to Ben not telling anyone he can trust about the watch, to see if something's wrong with it, as well as admitting he was really grounded.
In The Avengers: Earth's Mightiest Heroes, Wonder Man happened this way: Simon Williams goes to Tony Stark to demand to know why he's out to destroy his company, but Tony brushes him off and he storms out; this leads him to seek out MODOK and get revenge. They meet multiple times after Simon's been transformed into an ion-creature, but Tony never tells him that he only wanted to buy out Williams' company because it was failing financially, but he believed in Williams' scientific developments and wanted to work with him. He doesn't even tell the rest of the team until the end of the episode, after they've been calling him a heartless bastard for ruining Williams' life the whole time!
One of the many criticisms made to Ultimate Spider-Man is to contain several examples of this: at certain points, the characters end up in a situation that would have never happened if not for 1) Spider-Man making a stupid decision or 2) Nova being an idiot.
The two parter "Blade and the Howling Commandos" could have been over in 2 minutes. Upshot - Dracula wants a MacGuffin that will allow him and his vampire armies to walk in daylight. The catch: it comes in two pieces that have to be united for the effect to happen. Blade and SHIELD have one piece. There is then a big argument about who Blade's team will be to go and find the second piece...as opposed to just breaking the one piece right there and calling it a day.
Distressingly common in King of the Hill, where pretty much any character (Bobby especially) will believe anything any random stranger tells them and then stubbornly hold onto those beliefs in the face of all contrary logic or facts. The pilot episode has throwaway characters who appear for all of five seconds assuming Hank is physically abusive towards his family, and as such Hank spends the rest of the episode fighting off a clearly-unqualified social worker who is constantly trying to move Bobby in with a foster family. It gets worse when they start reusing the basics of this plot, when characters (most commonly Hank) are accused of things that are clearly out-of-character for them (again, most commonly by throwaway characters who only stick around long enough to make those accusations) yet everybody refuses to believe them when they deny the accusations.
Pigs is Pigs, a Disney short from 1954, is a shining comedic second-order Idiot Plot (as is the Ellis Parker Butler story it was based on). Basically, the recipient of a package containing two guinea pigs gets into a dispute with a slavishly by-the-book railway agent over the shipping rate — the agent insists that they are actualpigs, and therefore the higher rate for livestock applies to them. The customer, unwilling to pay the extra four cents, leaves in a huff, and the agent sends a telegram to railway headquarters to determine the rate, which is shown to be quite possibly the most inefficient organization in the free world. The mountains of paperwork the telegram requires are essentially mindless busy work for the staff, the board of directors argue and bicker and fight for days, and have to employ a fact-finding staff and a zoologist to arrive at the correct answer. Meanwhile, the package remains at the station, where the agent allows the guinea pigs to breed unchecked (and their reproductive speed is exaggerated to Tribble-like proportions). When their numbers start to crowd him out of the station, he tries to slow them down by playing them Irish jigs. Eventually, the station freakin' explodes. When the agent finally gets word of the verdict, he runs to tell the customer only to find that he's moved without bothering to leave a forwarding address. The agent is left with no choice but to return the guinea pigs to the railway headquarters — and the original two now have a million descendants and counting. Thus, a conflict that could have been resolved in less than a day with a simple trip to the library ends.
Speaking of a simple trip to the library, Flannery (the agent) is actually shown reading out of a book entitled "Care and Feeding of Guinea Pigs". If he'd bothered to read past "A cabbage a day keeps the doctor away", he not only would have found out how to sex guinea pigs, and that the only way to keep them from breeding is to keep the males and females separated, he probably would have read that guinea pigs are, in fact, rodents!
In Gargoyles, it isn't uncommon to any character (no matter how intelligent) to hold the Idiot Ball, but "Eye of the Storm" takes the cake in Idiot Plot. In this episode, Goliath and co. arrive in Norway in the winter, and needless to say, Elisa can't withstand the low temperature as well as her gargoyle companions. Suddenly, Odin appears in disguise and offers a coat in exchange for the "Eye of Odin" (a powerful magical artifact and Odin's ACTUAL eye). Goliath, knowing the corrupting power of the eye and finding the idea of anyone trying to bargain with the life of an innocent immoral, refuses the offer. Pissed off, Odin leaves and tried to challenge Goliath in the form of a polar bear. When it failed, Odin proceeds to kidnap Elisa and vows not to release her until he gets his eye back. Goliath, believing that Odin doesn't deserves such power, but knowing that he doesn't stand a chance against a deity, is forced to use the eye himself, turning into an avatar of Odin. While he gains enough power to face Odin and rescue Elisa, he also becomes mentally unstable and tries to protect his friends by abusing his powers and trapping them in a cave. Only when Goliath almost killed his daughter did he realize that he was going too far and relinquish the Eye. As noted by several viewers (and Greg Weisman himself), the whole conflict could have been avoided if Odin had been honest from the beginning, rather than acting as a divine dick.
According to Greg Weisman the reason for Odin's Jerkassery isn't so much celestial douchery as it is a combination of Values Dissonance and Poor Communication Kills. By the ancient customs of Norway, Odin's offer of a trade was quite customary (similar to a modern reward for returning missing goods. The eye is after all rightfully Odin's), but Odin had not been in contact with mortals for centuries. He assumed wrongly that such customs were still used, leading to the conflict. To his credit, Odin actually admitted to having done wrong and apologized for his actions.
In "Call Girl" Lois gets a job as a call girl on a phone sex line. Not only does Lois not recognize the very distinct voices of men she knows, nor do they recognize hers, but Lois actually does what the callers tell her to do instead of just saying she did.
Furthemore, Peter falls in love with Lois over the phone (not knowing it's her) and she gets mad at him for "cheating on her". Completely failing to realize that if she doesn't change her voice or personality over the phone, then DUH her idiot husband will fall in love with her all over again.
Almost every episode of the show (especially those revolved around Peter) revolves around this trope, usually completely intentionally.
Most of World Tour, especially the love triangle between Duncan, Gwen, and Courtney. Courtney generates nothing but blind hatred for Gwen, who never tries to apologize to her throughout all this despite clearly feeling guilty for kissing her ex-boyfriend, and takes most of the abuse sitting down rather than defending herself. Added on top of how Duncan gets off scot-free for cheating on Courtney.
In the Goof Troop episode "And Baby Makes Three," Peg is away from the episode. Everyone in the cast thinks Peg is pregnant and almost due. Goofy's an idiot, Max is an only child, and Pistol is the baby of the family, but Pete (who has had two children with her) and PJ (who has a several-years-younger sibling) really have no excuse. They also only believe this because they circularly take everyone else's word for it, including Pete taking Goofy's word for it and PJ taking Pistol's. PJ and Pistol also seem to think that a baby could do some or all of PJ's chores for him, and that Parental Favoritism only crops up when the baby is the same gender they are—despite the fact that their extremely imbalanced treatment was given extensive focus in the beginning of the episode. Then Pete is fooled by PJ's Paper-Thin Disguise despite the fact that a) PJ is clearly not baby-sized and has fully-developed teeth and b) the made-up explanation for where PJ went was delayed enough he should have caught on. Tropes Are Not Bad, as this is frequently considered one of the funniest episodes.
Several early episodes of The Dreamstone seem reliant on this, either due to the Urpneys' Villain Ball Magnet role or the heroes' Story Breaker Powers. "The Dream Beam Invasion" is a glaring example, when the heroes find out Frizz and Nug have shrunk into someone's dream, the Dream Maker briefly forgets he can just get rid of the bubble so the Noops can shrink in and submit them to a gratuitous Kick Them While They Are Down sequence as a Plan A for getting rid of them, which only wastes enough time for the Urpneys to grow back and become large enough to scare away the Noops. Basically the heroes pulled the most evident Villain Ball in the show.
The common criticism towards Young Justice is that the majority of The Light's plans wouldn't have succeeded if the heroes of the show had acted just as intelligent or competent as The Light.
One episode of My Gym Partner's a Monkey involves Slips, the snake, shedding his skin. All the other students in CDMS are going through various changes involved with adolescence (such as Adam growing a chest hair). As a result, everyone believes Slips has a contagious disease causing them to go through these changes, and then they start a riot, causing Principal Pixiefrog to intervene and show them a video teaching them about growing up. The fact that none of the students were aware about growing up and blamed Slips for it places it into this category.
The entire second season of Frisky Dingo qualifies, because of the presidential election arc. Nobody seems to realize that Xander and Killface both fail to meet constitutional requirements: Xander is younger than 35, and Killface is not a natural-born US Citizen. They're eventually called out by a Pompous Political Pundit, but when it's too late to change the ballot.
The entire premise of Wolverine and the X-Men revolves around the idea that Xavier would trust Wolverine, a grumpy, burned out, roughneck loner with damaged memories and possible latent sanity issues, with the responsibilities of leading the X-Men. And that's BEFORE the X-Men all get kidnapped by ninjas.