In the Ben 10 episode "A Change Of Face", Grandpa Max doesn't notice that Gwen is acting odd, the reason being that villainess Charmcaster has swapped bodies with Gwen. Even later on, after the ruse has been revealed and more body-swapping has occurred, Grandpa Max still can't tell who's who without a scorecard... This is pretty glaring, given that Max is a former plumber (this show's The Men in Black equivalent), has been repeatedly shown to be pretty clever, and above all is their grandfather.
Another example is in the Ben 10: Alien Force episode "Good Copy, Bad Copy". You'd think that, considering how much weird stuff they've seen, Kevin and Gwen would immediately be suspicious of "Ben" (really a Galvan named Albedo) claiming he's looking for Ben. Instead, they just assume he's the real thing and has lost his mind, setting up for the old "Which one is which?" bit.
Also "Duped" from Ben 10: Ultimate Alien. With Ben splitting into three of himself, each a different aspect. Sending the sensitive part to fight Forever Knights and the Jerkass part to Julie's tennis game is an idiot ball the size of Texas.
In the Gargoyles episode "Vows", Demona gains possession of an artifact known as the Phoenix Gate, which allows the holder to travel to any place at any time at will. She travels back in time and informs her past self (also in possession of the past version of the Gate) that SHE should use the Gate to change history, instead of just doing it herself with the Gate she already possesses. The ultimate lesson is that history is immutable, though the reason for this apart from a large, conspicuous Idiot Ball is unclear.
Made worse by the fact that present day Demona says she remembers the whole incident (well, Goliath's talk after she got knocked out), but if she remembers that, it's pretty odd to think she'd have forgotten meeting herself and seeing her other self's defeat. So if she already had memories of her plan failing, why go through with it? It reeks of Dr. Manhattan style pre-destination.
The aforementioned pales in comparison with the sheer idiocy Demona displays in "Hunter's Moon". Goliath and Co. break in when she's about to deliver the fatal blow against humankind and unleash the enchanted virus lethal to all sentient beings (Gargoyles will be protected by a magical figurine she has on her table). Not only does Demona start monologuing, in the worst Bond Villainesque manner possible, but she actually POINTS at the figurine as if asking Goliath to smash the thing. He obliges.
Not to mention the unpondered fridge-horror: Demona spends her days human. Demona is also nigh-immortal. Best case scenario, the virus 'counts' in terms of her curse about Macbeth, and killing him disperses her nigh-immortality, allowing her to die. Worst case, it doesn't count and she spends her days dying repeatedly and her nights in agony from Mabeth dying repeatedly.
This is actually an explicit feature of Demona's character. She's such a mess of contradictory issues, topped with a healthy dose of repressed self-loathing, that she has a noticeable tendency to sabotage herself. Word of God even outight says that Demona is, and always has been, her own worst enemy.
In "Deadly Force" Elisa Maza, an experienced police officer, is apparently stupid enough to leave her gun lying around her apartment loaded, which leads to Broadway accidentally shooting her when he starts playing with it.
in G.I. Joe, the creation of Serpentor was likely the biggest mistake in Cobra's history, but one part of it stands out as especially idiotic. After Dr. Mindbender fails to obtain the DNA of Sun Tsu (meant to give Serpentor wisdom and patience), he thinks that replacing it with Sergeant Slaughter's DNA would be a good idea. (Sun Tzu, Sgt. Slaughter. Same thing, really) Ironically, Cobra Commander (whose incompetence is the whole reason for the plan) is the sole voice of reason when this suggestion is made, ranting about how ludicrous it is to put the DNA of an enemy into someone who is meant to lead them. The point is eventually rendered moot, as Slaughter himself destroys his own DNA sample just as the experiment is happening, and Serpentor is born without it; without either Sun Tzu or Slaughter's DNA, he proves to be impatient, impulsive and little better than Cobra Commander.
Batman: The Animated Series: Harley Quinn's idiot ball is The Joker. With him around, she's hardly more dangerous than the average Mook. When she sets out on her own (Or with Poison Ivy), she becomes a deadly threat that on one occasion completely outsmarted Batman and came closer to killing him than any other villain in Gotham.
Another example involving Grodd occured at the conclusion of the Justice League episode "Secret Society". After all the other villains have been defeated, he tried to fight Superman... By himself. The end result was both painful and humiliating for Grodd.
Possibly justified in that this was a last resort and originally Grodd was trying to escape. And with his mental powers he came close to overwhelming Superman.
Kim Possible: Ron's intelligence varies from episode to episode, and sometimes he's just flat out clutching this ball. Kim gets it a few times, too.
Ron: "Hello, Information? I'd like the number for 911 immediately please!"
Honorable Mention: The Orb of Confusion — a literal Idiot Ball (pictured at the main page).
The trope applies to several episodes, with varying results - SpongeBob, Mr. Krabs, and/or Plankton will be saddled with the Idiot Ball at any time whatsoever. The only ones safe are Sandy and Squidward... and even then...
The episode where SpongeBob lost his name tag had a big one. Normally, retracing your steps is a smart way to find something you lost. The idiot part is how SpongeBob thinks he has to perfectly recreate everything he did that morning, no matter how unpleasant, and having to redo the whole thing if even the tiniest detail is missed.
Squidward has been known to hold the Idiot Ball. In "The Snowball Effect", Squidward tells Patrick to think of him as SpongeBob, to teach him how to have a snowball fight (while Spongebob asks to be Mr. Krabs). Squidward throws a snowball at Patrick and says, "Now, what are you going to do?" Patrick throws a snowball back at Squidward, who had said not five seconds ago to think of him as SpongeBob. Squidward asks why Patrick didn't throw it at SpongeBob.
The other time his stubborn, outspoken nature came back to bite him was in the first movie, where he brags to Plankton that he's going to expose him, and doesn't get very far.
The same goes for The Fairly OddParents, where in most episodes it's Timmy's idiocy that gets the plot moving. And very often keeps it moving, as Timmy could easily resolve most of the plots with wishing, and on the occasion that there is a reason why he can't, it's usually pretty flimsy.
In "Man's Worst Friend", Timmy doesn't seem to immediately notice that Sparky has been replaced with his anti-fairy counterpart. Keep in mind that the main distinguishing feature separating anti-fairies from fairies is that anti-fairies are entirely blue.
The Flintstones: Fred Flintstone gets hit with one halfway through "Dino Disappears". When he's looking for his runaway dinosaur Dino, he sees a random dinosaur in someone's backyard that looks just like his pet and immediately decides that this must be Dino, to the point of ignoring all evidence to the contrary and enacting out a kidnapping scheme. What makes this more noticeable is that, not five minutes earlier, Fred himself encountered three dinosaurs that looked exactly identical to Dino but weren't him.
Iroh, usually Retired Badass and The Caretaker in one, once almost killed himself by drinking tea made of the leaves of a perceived delicious tea plant that wasn't (well, he and Zuko desperate for food at the time). Followed by a small Find the Cure plot. (Though this may have been a bit of character exposition, meant to demonstrate exactly how much Iroh enjoys his tea.) However, this also could have been a Batman Gambit on his part. As up until then Zuko refused to go to a village for help, and Iroh did this to make him decide to go.
Also, using Firebending to heat his tea while they were trying to stay incognito... in the Earth Kingdom... surrounded by refugees from the invading Fire Nation forces. Zuko immediately lampshades: "What are you doing firebending your tea?! For a wise old man, that was a pretty stupid move!" Iroh doesn't make that mistake again, though- a later scene has him borrowing spark rocks for their stove even though as far as he knew nobody would see him lighting it himself.
Taking the two above, it looks more like his dangerous obsession with tea is his own idiot ball.
Katara, who before had been hiding out of sight, deciding to step out and into the crazed Azula's line of vision just as Zuko has taunted her to shoot him with lightning. This is just so Zuko is knocked out of the fight protecting Katara so she can defeat Azula.
Also Zuko himself. Already having an edge over a crazed Azula, it was looking like he could win the fight through normal firebending alone, but then feels the need to taunt her into using lightning. And he doesn't just taunt her, he specifically says "Whats the matter, afraid I'll redirect it?". Even a VERY out of it Azula isn't stupid enough to shoot lightning at a guy who specifically says "I can stop lightning".
Professor Zei. Wan Shi Tong is taking his library back to the Spirit World, feeling betrayed by humans' use of its knowledge for selfish and destructive purposes. He has one and only one chance to escape, but his obsession for knowledge leads him to stay in the library as it sinks into the other realm, in a place not designed to be inhabited by mortals. He has essentially condemned himself to death. Much later in The Legend of Korra, his withered skeleton turns up in the library, though the exact cause of death is not confirmed.
In the Sonic animated series, Antoine would occasionally be used for this. The mini-episode Fed Up with Antoine was the most blatant example of this trope.
In Codename: Kids Next Door there's Numbah 86. While it may be easier to list the appearances she made where she wasn't an idiot, "Operation: E.N.D." stands out. As Numbuh One tried to tell her (not that she listened) it should have been obvious to her that he was not thirteen years old, given that the two of them were in kindergarten together. Apparently, she believed a computer over something she had personally witnessed. However, she was a serious feminist, so maybe she just was using it as an excuse to get rid of him...
The placement of said Idiot Ball in that case may have been a slight homage: 86 also being the Agent number of the Cloud Cuckoolander lead in Get Smart.
In one episode, Lisa is no less smart than she usually is, but she's feeling like an idiot because a new girl in class, Alison, has proven to be better than her at everything. Visiting Alison's house, Lisa attempts to play an anagram game with Alison's father but fails miserably. Taking her to be a simpleton, Alison's father hands Lisa a red rubber ball, saying "this is a ball. Perhaps you'd like to bounce it."
"Pranksta Rap". The plot revolves around Bart faking his own kidnapping and gives rise to two idiot ball moments. Bart handwrites the ransom note. Marge fails to recognise her own son's handwriting.
Star Wars: The Clone Wars has an episode where Obi-Wan and Anakin are sent to negotiate with some Weequay who have captured Count Dooku. Right from the start, they go in expecting a trap, yet apparently did not bother to think of any countermeasures. They quite willingly hand over their lightsabres rather than hide them and worst of all, at the end when they've finally broken free and have the Weequay leader at blade point, ready to give up and come to prison quietly, Obi-Wan tells Anakin to let him go. Why? Why? Why? They never even try to explain why Obi-Wan suddenly wants to let someone who lied to them, drugged them, kidnapped them, and tortured them get off scot free!
Also, once Obi-Wan and Anakin were drugged, they seemed to magically forget that Jedi have poison-neutralizing powers.
The Clone Wars also has Cad Bane, a bounty hunter who seems to have the power to hand out idiot balls to all of his enemies. In every one of his appearances, he manages to succeed by turning the Jedi into complete morons. Bigger idiots than they usually are in the prequel era, that is.
The entire clone army seems to be equipped with standard issue idiot balls. While it is reasonable for the droids to not really have any sort of care for self-preservation, the clones will often completely ignore cover and tactics to charge enemies head-on.
That part's taken straight from Attack of the Clones. For such supposedly highly trained troops (which being heavy hammered on multible occasions ) they rarely seem to exhibit even basic common sense, much less any signs of military tactics. It's just mostly "let's charge on foot through flat, open ground!"
MaceMother FuckingWindu starts off with the idiot ball in the Zillo Beast arc of Star Wars: The Clone Wars. A giant beast that eats the inhabitants of the planet the Jedi are negotiating with is found, and Mace Windu turns into Captain Picard, demanding that the rights of the creature be respected, and it not be killed. The native Dugs aren't too thrilled by this, as this thing is the equivalent of Godzilla on their world, and demand that the Jedi help kill it, under threat of ceasing negotiations with them. Mace Windu explains this to Senator Palpatine, and asks to be allowed to take the beast off world. Senator Palpatine says to defer to the locals if the creature's death is a sticking point. So now we have a conflict between killing this innocent creature and signing a necessary treaty, or letting it live and losing out on valuable resources. Only, it's completely unnecessary: let's overlook the observation that the Knights of the Jedi Order are not members of the United Federation of Planets, and while not bloodthirsty, have never shown an aversion to attacking creatures, innocent or not, who were trying to kill them. Windu presents the issue of the creature's survival to the Dugs as one of just "letting it live and putting up with it", while he presents the option of having the creature taken off world to Palpatine. The Dugs have a perfectly legitimate reason for not wanting a creature that could destroy a large part of their population to be allowed to live on their planet. However they might be more apt to let it live if Windu were to actively present the removal option to the Dugs. Since "dead" and "on the other side of the galaxy" have the same end result from the Dugs's point of view, there's no reason why this couldn't have been even floated as a solution, thus ending about half an episode's worth of needless conflict.
Every character in the original Teenage Mutant Ninja Turtles cartoon would carry the ball when the plot required. A stand-out moment includes a scene in the series' third episode, when, left to their own devices in April's apartment, the turtles suddenly become retards: Leonardo begins throwing tubes of lipstick at a painting, calling it "target practice"; Raph interprets "insert capful of Shampoo into tub" as referring to an actual baseball cap; Donny starts fiddling with April's answering machine with no regard as to her privacy; and Mikey proves incapable of making instant pizza which he had no permission to touch. April is understandably furious.
The Transformers: Optimus Prime gets whacked hard in the face with this in the episode "Heavy Metal War". He's been fighting Megatron for centuries, long enough to know which powers Megatron does and doesn't have...and yet he doesn't realise that Megatron cheated during a one-on-one battle until Teletraan-1 spells it out for him. Even though Megatron uses powers that he has never used before.
Thrust is actually pretty smart in Transformers Armada when he first shows up. After several Deus ex Machina-induced failures he starts spending a lot more time around the Idiot Ball, culminating in getting his rear kicked by human children.
In a Justice League Unlimited episode, you have rebelling young twin heroes, one of whom can turn into different animals, real or not, and the other can turn into water. When these two heroes (and other young, misguided heroes) try to blow up the base they live in after thereveal, a group of Justice Leaguers tries to stop them. The twins attack a member of the Justice League by drowning him in a room filled with water and have a T. rex, obviously not an aquatic creature, attack said hero, underwater. Again, this happened underwater. Who's the Justice Leaguer they fought? Aquaman. Though the characters were in the middle of a mental breakdown, so thinking clearly was not exactly something to be expected. If they're inspired, as it seems likely, by the Wonder Twins of Superfriends, then one could say they keep pretty true to the original characters.
Teen Titans hands Robin a huge idiot ball at the end of "Trust". After spending an episode fighting against a shapeshifter who had both shown that she could mimic both the appearance and voice of anyone, cannot replicate powers, and loses her structural integrity when she's exposed to strong heat. What does Robin do after a fight where the result isn't clear? Immediately trust the Not Hot Spot, and handed over a spare communication device - which allowed The Brotherhood of Evil to track down every. single. superhero. that the Titans gave the com device to.Nice Job Breaking It Robin. This makes it worse since Robin is supposed to be the smart, suspicious one.
Inverted by Inspector Gadget, bizarrely enough. While Penny and Brain were typically the ones who saved the day, there were quite a few isolated moments when Gadget himself could actually show competence when the plot demanded it.
Episode 12 of Sym-Bionic Titan has some Idiot Ball moments when you consider two things: One, why didn't Lance and the others consider stomping and or obliterating the supposedly deadmonster's body just to be safe. Two, wouldn't it have occurred to Ilana and Lance by now that Octus isn't a normal robot considering his build and powers? It's made clear early on that everyone on the show occasionally picks up the ball for the plot (episode 4, anybody?).
The Animals of Farthing Wood: Weasel conveniently forgets Fox's message for Adder to kill Scarface and mixes it up as simply killing a blue fox, despite she being the one who suggests they get Adder to kill Scarface.
Every mane character in My Little Pony: Friendship Is Magic holds the Idiot Ball at some point, no exceptions. Even characters that would formally know better get turns to be a silly pony (usually in their own limelight episodes for spotlighting the character's personality flaws).
This can be justified if you consider that one of the main goals of the series (at least in season 1) was to teach young girls about common mistakes, so that they can avoid them and become better friends.
Special mention goes to The Cutie Pox, since Apple Bloom's entire family and everyone at school doesn't show even the slightest concern that she's been endowed with two cutie marks. That's the equivalent of someone's child growing an extra arm, and everyone just saying "He'll be a great piano player!"
The second season finale has everypony play a game of "toss the idiot ball", with Celestia tossing it to Twilight, Twilight tossing it to her friends, her friends tossing it to the local Big Bad, and the Big Bad simply taking the ball and running home with it. To elaborate: Celestia responded to a threat by increasing security for a wedding and tensing up Twilight, who then proceeds to accuse the bride of being evil based off her jerkass tendencies. This then led to her friends walking out on her despite a few factors that should have convinced them otherwise (the fact that she was a jerkass to them is one), which leads to the bride launching an all-out invasion now that her cover is safe... Unfortunately, she got handed the ball and caused mistake after mistake to happen.
Spike holds it in Inspiration Manifestation, since it's fairly obvious Rarity has gone mad with power, but decides to not tell anypony about it because he's afraid that Rarity won't consider him a friend anymore.
The fourth season finale has just the Princesses playing around with the Idiot Ball while the premiere just had Twilight's friends toss it around. Letting Discord, an untrustworthy Trickster God, to go nab the Big Bad unattended led to his Face–Heel Turn in the former, while Applejack led away the only thing they need to stop the thorny vines from attacking Ponyville in the latter.
The Cutie Mark Crusaders appeared to have held it when they were on the lookout for their cutie marks. Their talents were hinted at by Twilight Sparkle in Season 1's "The Show Stoppers" but they tried for another four seasons to get them by doing other things. However, this turns out to be subverted when they finally get them in the Season 5 episode "Crusaders of the Lost Mark" since their symbols turned out to be different than what they expected.
In Voltron Force S2E1 (Inside the Music), Pidge picks up the Idiot Ball and runs with it for a touchdown. He's so heavily invested in keeping his secret identity as the mastermind of the band "Stereolactic" a secret, that he doesn't even tell his True Companions. This results in the Cadets wasting time and effort chasing him down, believing that he's the Drule agent, that could have been used finding the real agent.
Most of the characters on Futurama, save for Hermes, will carry the idiot ball from time to time, but Leela, often times, will take off running with it. To analyse this for a second, Leela is the Only Sane Woman most of the time; the character with the most common sense and intelligence (combined; Farnsworth might be the brightest intellectually but he's barely sane). This makes it difficult for the writers to start up certain plot points without taking a few IQ points off of Leela for the episode.
In Strawberry Shortcake's Berry Bitty Adventures, the titular redhead is, if not the smartest character in the show, the most consistently competent. Yet in "Star Light, Star Bright", Strawberry asked the pop singer Cherry Jam her name when she first met her. That was despite Strawberry earlier mentioning that Cherry Jam was her favorite singer, Cherry repeatedly singing a line from a song she was writing when Strawberry found her (which Strawberry helped with by singing her suggestion back), Cherry Jam being dressed in similar clothing as she was in the "music video" at the beginning of the episode (including trinkets with cherries on them), and Strawberry noticing that, near Cherry, it smelled like cherries. Four-year-olds facepalmed watching that.
In Ultimate Spider-Man episode "Venomous", Peter Parker hides from his other team members and S.H.I.E.L.D. that Venom, the monster they have been chasing, is actually a possessed Harry Osborn. This allows his team to completely trash it and allows Spider-Man to constantly botch and drag out their battles with it...rather than just have Peter tell the people that could have helped him sooner by holding back and using their infinite scientific resources to cure him sooner.
While a lot of it is Flanderization into simply dumber characters, South Park is generally willing to hand out idiot balls for whatever reason they feel like it. Given that the show has no shortage of idiotic characters, it manages to have many Idiot Plots without handing out any balls, but occasionally Stan or Kyle, the only sane kids, will start acting like idiots. Before the majority of the cast became idiots normally, idiot balls were more common, and their frequent usage is likely what lead to flanderization.
Ms. Marvel picks one up when she sees what appears to be the rest of the Avengers walk out of a Skrull spaceship. Despite being well aware of the Skrulls' ability to shapeshift, she's only suspicious for as long as it takes for "Tony" to give a made-up alibi. She only starts to show signs of knowing once they start invading Wakanda for no reason, and even then she needs to see "Thor" die and revert to his green alien form to be 100% sure.
The Nickelodeon Christmas Episode of Doug gives the entire town of Bluffington an Idiot Ball. In the beginning of the episode, Porkchop is shown accidentally injuring Beebe Bluff in an attempt to save her from drowning in a frozen lake. Because of this, her father initiates a massive smear campaign against him, painting the dog as a monster... and the town believes this, even though many episodes show Porkchop helping people. Even worse, when Doug tries to start a petition to save him, they flat out dismiss him, saying that "It's Christmas." When Porkchop goes to trial, Doug is forced to call everyone out on it.
Lin is suddenly incompetent and quite frankly lazy in her handling of the attacks on Republic City, whilst constantly berating Mako for actually being proactive. She even falls for a rather obvious frame up when Mako gets too close to the truth. This was not received well by fans.
Korra enters the spirit world to close the portals to the human world when in reality it would be much safer to wait until after Harmonic Convergence has ended. Only she can open both portals after all. And in the end the two Big Bads take a child hostage to force her to do so. This culminates in Vaatu's escape when the convergence arrives a week later.
Suyin is guilty of this in season 4, where her attempt to kill the antagonist, Kuvira, just makes things worse and lands her and the bulk of her family in jail.
This happens quite a bit in Total Drama for when a character gets themselves eliminated. There are many instances where, in order to eliminate a contestant, said contestant will make a stupid mistake that will cause them to get either blindsided or temporarily earn the hatred of their competitors. An infamous example is when Heather hides the invincibility statue in All-Stars after finding it, only for Alejandro to find it and use it against her.
Mickey Mouse, Minnie Mouse, Donald Duck, and Goofy are all handed the Idiot Ball in the 2014 Paul Rudish short Mickey Monkey. After a monkey steals Mickey's clothes, Minnie, Goofy, and Donald all mistake the monkey for Mickey and vice versa even though the only way they are identical is that they both have black fur and a similar body shape. They don't even suspect anything when the monkey doesn't speak English or even when he grooms Minnie as he would for a mate. After Mickey gets his clothes back and chases the monkey away, he is then unable to notice that now his friends have been replaced by a trio of gators that stole their clothes as the monkey did with him.
In the ChalkZone episode "Double Trouble", both Snap and Penny seem to be holding it when they run into Rudy's evil robot double created by Skrawl and Craniac 4. While Robo-Rudy has some pretty clear differences between him and the real Rudy, both Snap and Penny (who's The Smart Guy, even) can't tell the difference until Craniac 4 slips up his dialogue for Robo-Rudy and accidentally reveals he's a robot double.
Phantom Limb was holding it very firmly in The Venture Bros. during The Monarch's backstory. He fails to recognize Monarch as one of his own Shadowmen when he is caught with Queen Etheria (who would go on to become Dr. Girlfriend) and buys his story that he's actually a different villain named Manotaur.
Family Guy has nearly the entire cast written as idiots and most episodes involve an Idiot Plot. Certain characters like Brian and Joe Swanson tend to be used as the voice of reason, but they are sometimes given the idiot ball to help advance the plot or assist in the random gag of the day.