The entirety of The Grim Adventures of the Kids Next Door was just one big Idiot Plot that involved all kinds of KND and Billy and Mandy characters falling for some of the most pathetic Paper Thin Disguises in fictional history. We can expect this thing from those minor Billy and Mandy characters but every single KND moon base operative too? The only ones without an Idiot Ball attached were Mandy, Grim, Numbuh Five, The DCFDTL, and (most of the time) Numbuh One. 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 And 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.
You'd think 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...
If they don't know who's behind the mask, why would they assume the Monster of the Week isn't dangerous?
Some incarnations (particularly Mystery Inc.) justify this by making the monsters total psychopaths who are perfectly willing to commit murder and widespread destruction if someone gets in their way. They may be frauds but that doesn't mean they're not dangerous.
Why does the Scooby gang spends hours running around looking for clues as to who the monster is instead of just setting up a trap and capturing the guy right at the start? Just Eat Gilligan, that's why.
To answer the original question, it's simple: whenever they demonstrate awareness of the fact that they are the Trope Namer for Scooby-Doo Hoax, they find out that they are in one of the movies where the monsters are real. Pretending to fall for it just might be the smartest thing they could do.
Happens more than it should to poor Eddie Spenser, Jr. on Filmations 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. Some 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 and, in the Super Mario World cartoon, they get forced to jump into a warp pipe leading to a Magikoopa's haunted house HQ when he threatens to throw Bob-ombs at them"!
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.)
What's with Andy? is a walking Idiot Plot as you'd have to be a complete idiot to fall for ANY of Andy's pranks.
In 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.
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.
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 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 blocks the counter-wish.)
One of the worst specific example would be the episode where Timmy joins the circus. 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!"
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 JLU 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 shape shifting, 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 Simpsons had an episode called Bart Sells His Soul, about Bart and Milhouse arguing over whether a soul is real or not. Then Bart "sells his soul" to Milhouse (which was really a piece of paper with the words "Bart Simpson's Soul" on it) for 5 dollars and at the end when Lisa buys Bart's "soul" back with the change in her piggy bank, Bart eats the paper.
The first movie 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 in Futurama are deliberate Idiot Balls. The ones that aren't usually end up getting plenty of crowning moments of stupidity too.
Leela (checking e-mail): Spam, spam, junk. (GASP) The very last pygmy rhino is going extinct?! Unless it gets my credit card number?!
The third Futurama movie, 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 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.
TaleSpin despite being a great show has a couple of these.
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.
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 forDib 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.
Every episode of Beavis and Butt-head is this, being that the titular characters are incredibly stupid, and the rest of their town isn't too bright either.
A lot of the episodes of Ren And Stimpy end up being this, especially later on in the series.
'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.
My Little Pony: Friendship Is Magic: Pretty much the entire episode "The Super Speedy Cider Squeezy 6000". The presented "happy" ending of the episode was not anywhere near the best solution, but no one ever gave a second thought to anything.
Granny Smith was the one to start the whole plot, by agreeing to a competition where the Apples had nothing to win and everything to lose. Flim and Flam could not have forced them to participate, and without access to any apples, could not have competed with their cider-selling business either.
Alternatively, since the titular machine could produce cider three times as fast as the Apples doing it by hoof, even the agreement where only a 25% cut of the profit went to the Apples would have given them 75% of their previous income - which could have been in addition to profits from the smaller manual cider production that composed the only income they had before, and so could have ended up with a full 175% of their previous income.
Flim and Flam, after ditching the quality control of their cider and winning the contest as it was only about volume produced and not quality, started off by selling the cider they made after the decrease in quality, leading to their fall. Had they sold only the barrels with the cider that was implied to be at least as good as the Apple's production, they would easily have kept both their monopoly on selling cider and free access to enough apples to meet Ponyville's increasing demand.
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 Spongebob Squarepants 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 Tokuster, causing his father to assume that Azmuth died, causing him to attack Ben, whose watch absorbed the Tokuster. 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 several points, the characters end up in a situation that would have never happened if not for 1) Spider Man acting like either an idiot or a troll 2) Nova being an idiot.
Distressingly common in King of the Hill, where pretty much any character (Bobby especially) will believe anything any random stranger tells them. 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 are accused of things that are clearly out-of-character for them 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 form of a polar bear. When it failed, Odin proceeds to kidnap Elisa and vows not 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, 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 recognise the very distinct voices of men she knows, nor do they recognise hers, but Lois actually does what the callers tell her to do instead of just saying she did.