In the final arc of the original Digimon Adventure, Myotismon was just defeated, but Piedmon has taken over the Digital World, which means the 7 kids plus Tai's kid sister Kari, the 8th Child, have to go and save it. Things go From Bad to Worse faster than a Digivolution when several Digimon they've met die one by one, and Matt gets pissed at Tai's Determinator attitude and runs off when Biyomon Digivolves instead of Gabumon while trying to save his kid brother T.K., and ends up falling for the lies Cherrymon feeds him about his relationship with Tai, which leads to Agumon and Gabumon being reduced to their In-Training forms after some in-fighting, and even after making up, Matt still leaves (with Mimi and Joe, who leave for more believable reasons).
During the Land of Waves arc, Sasuke was having a hard time inside Haku's Hall of Mirrors. All that Naruto had to do to was to attack the Hall of Mirrors from the outside. Instead, he chooses to go inside the Hall of Mirrors, and complicating the situation further.
Later, during the Chunnin exams in the Forest of Death, Sasuke gets hit by the Idiot Ball as he agrees to Orochimaru's offer of giving up the team's scroll in return for their lives, and Naruto calls him out on it.
During the invasion of Konoha, Orochimaru is in a position to kill the Third Hokage with a kunai. Instead of doing that, he backs off and summons reinforcements and makes a few long-winded speeches. The battle doesn't end well for him.
All the filler episodes of Naruto seem to involve a multitude of Idiot Balls as Rule of Funny. In one episode (161), Naruto fails to recognize two people wearing obvious disguises to make them look like Guy and Lee, and misinterprets their increasingly deviant tasks as "taijutsu" training. In another (193), Lee fails to recognize his own sensei (who he idolizes) because of a weird wig.
Subverted when Naruto goes on a "training mission" with fellow Jinchuriki, Killer Bee. The mission is actually a ploy to keep Naruto away from the Big Bad, who needs to sacrifice Naruto to complete his plan. Kabuto launches an attack on the island where Naruto is, but Naruto mistakes the explosions for... shifting seas?
Sasuke dribbles the Idiot Ball for a bit, his goals switching from "Kill people who were responsible for my childhood trauma" to "Kill everyone who lives in the same area as the people who caused my trauma", all because a man in a mask told him so, while said man also wasn't denying his own hand in causing said trauma.
The Uchiha Massacre was part of Itachi's plan to keep Sasuke loyal to the village. He wanted Sasuke to become stronger and to hate him, and him alone. To be thorough, Itachi Mind Rapes him, twice, leaving him comatose the second time, and mocks him for being weak both times.
Subverted when Sakura discovers that Sasuke's going to abandon Konoha, so rather than report this to the proper village authorities, or find Naruto or some of their friends to back her up, she goes to him, alone, and pledges her love for him, hoping to change his mind. She is 13 years old, but she is also a trained shinobi, and Sasuke was practically moseying out of the village.
Konoha's handling of their two most prized, plot-relevant assets: Naruto and Sasuke. Naruto holds a powerful demon inside him, and Sasuke is the last loyal Uchiha, with the ability to copy techniques. Rather than train them in secret, they both attend public school. Naruto lives alone when he's introduced in the story and doesn't make any friends, and isn't put into any sort of mental therapy. Sasuke gets infected by a known terrorist, but rather than monitor him closely, they leave him mostly unattended, and he manages to walk out of the village with no one but one of his closest friends knowing about it. Naruto is allowed to leave the village perimeter with Jiraiya as his only bodyguard, despite the presence of a terrorist cell who want the Nine-Tails, and the village keeps zero tabs on Naruto.
Karin is in love with Sasuke, even during his Moral Event Horizon. After he stabs her in the chest and asks a passerby to execute her, she returns to him, pissed off, but almost instantly forgives him for it. This would be passable insanity if she weren't already established as reasonably intelligent and focused enough to be a prison warden. She's so lovesick it blinds her.
Asa from SHUFFLE!!. This requires some backstory. Her mother, Ama, is a homunculus, and had suffered greatly due to magic when she was younger. Although she now lives happily with her daughter, Asa inherits no small amount of magic as a result. Unused, this power will wreck havoc on her, and eventually lead to death. All she needs to do is occasionally use slight magic to stay healthy, but refuses to do so, since she doesn't want to remind her mother of her tragic past. Let me rephrase this. In order not to upset her mom, she decides to subject herself to a long, painful, completely preventable death in a hospital bed while her organs systematically shut down. While her mom, who knows she has magic anyway, cries at her bedside. It takes her friend attempting suicide to force her to save her own life.
Ranma from Ranma ˝ (at least in the manga) is normally the The Ace combined with minor Guile Hero. Able to lie and cheat and even try to find things to use against others. An example would be going to Nabiki's room to find things to manipulate her. Of course there were times when he fall and stumble due to his morals and therefore has the most Butt Monkey moments of everyone. In the Gambling King Arc, he suddenly becomes stupid. Unable to keep his face straight? Easily trick by the idiot Gambling King. It goes against everything shown in the previous stories. Thought people familiar with the anime will be at home in this Arc, due the anime Ranma being much closer to a savant or Idiot Hero.
Irina Woods from Mai-Otome is normally a smartgirl, but why in the world would she lend her library book on "Human Engineering" to Tomoe when she could have easily returned it herself? Any reasonable person would have suspected Tomoe was up to no good after she returned alone from her little "chat" with Miya, and cautiously refused to turn the book over to her. The lesson learned here, kids? Keep an eye on your own belongings!
By giving the book to Tomoe, Irina's just passed the Idiot Ball, as Tomoe leaves the book behind in Professor Youko's lab in a presumed attempt to frame Irina for injuring Erstin, despite the fact there were at least a dozen eyewitnesses who could vouch for Irina never having left the classroom at the time.
Luckily for Irina, the professor recovers the book in a later episode and returns it to her, no questions asked.
Pokémon: Ash Ketchum in the Diamond and Pearl series. It'd be impossible for him to lose at this point, seeing how he's had his Pikachu through three different regions (four, counting the Orange Islands), let alone all the other Pokémon he's acquired and trained, on top of his extensive knowledge of Pokémon battling and counter-typing from his years of battle. So what does he do? He loses against the first gym leader in Sinnoh.
Actually, who's to say his opponents haven't had similar adventures? Back in Kanto, Ash mentioned it took roughly a year for him to come back to Viridian City, meaning it takes him around a year to complete each journey, give or take a month or so. That would only make Ash around 14-15? There are still people far older and more experienced than him and his handful of years.
Best Wishes! spoofs this theory by defying all non-relativistic laws of time: Ash is still ten. Even though based on earlier dialogue AT LEAST one year must have passed.
A specific example: The Whirl Cup. Ash is facing Misty's Psyduck with his Kingler. He already demonstrated knowing how to bypass Psyduck's headache powers in a previous episode, yet the writers made him fall for it just so they could hand Misty the win.
In a later episode, James is feeling down because his Cacnea is having trouble learning a new move. Ash and co., for absolutely no fathomable reason, decide to help train it. That's right. They're willingly going to help Team Rocket, the group who have been stalking them, spying on them and attacking them since day one, get stronger. When Dawn points out that Ash is being a monumental idiot, he brushes it off, saying it's their job to help Pokémon. Have to wonder how he would feel if the training had succeeded and Team Rocket used their newly improved Cacnea to successfully steal Pikachu once and for all...
Team Rocket actually grabs the ball constantly. Among their exploits are running straight at a Frontier Brain while yelling at him to hand over the Articuno that was allowing him to use it in battle (without sending out any of their Pokémon).
And then we have Giovanni, in the first movie. He's been using Mewtwo as his servant for a while, pretending that he believed Mewtwo could be equal to humans. He must have grabbed the Idiot Ball when he decided to let Mewtwo know that he felt Mewtwo was only meant to serve humans... Giovanni isn't unwittingly saying this to a Pokémon that he doesn't know is powerful. He's seen Mewtwo at work. He knows what devastation his "servant" can cause. Even with the armor. Giovanni knows that Mewtwo is an enhanced clone of Mew, the probable ancestor of all Pokémon. And he still insults Mewtwo, right to his face. With a leader this dumb, it's a miracle that Team Rocket hasn't all been rounded up and arrested, or killed by angry Legendaries.
Specifically, this is the Pokemon who, minutes after "birth" slaughtered every last scientist in the facility, burned it to the ground and, under Giovanni's training, grew STRONGER still. And AFTER all this, Giovanni STILL chases Mewtwo to the next region over, bringing an army to try and stop him.
In Giovanni's defense, he grew a few brain cells and was much more prepared in dealing with Mewtwo and might have succeeded if Ash and his friends hadn't come to Mewtwo's rescue.
This could apply to Mewtwo as well, as he erased everyone's memories of his existence, but for some reason it did not occur to him that he forgot someone, and it nearly cost him his life. Though to be fair, he might've thought Giovanni blew up with the entire Rocket base and presumed him dead, but he never bothered to check.
Omi Tsukiyono from Weiß Kreuz, who is supposed to be by far the smartest member of his team of Hitmen with Hearts, gets hit by the Idiot Ball when he decides that the best thing to do with a CD that causes suicidal psychosis in half the people who listen to it is play it and see if it works. It takes two of his teammates passing out for him to realize what a very, very stupid idea this was.
Suzaku and Lelouch from Code Geass strike a deal, wherein Lelouch has Zero's crimes absolved in return for exile from Japan, as well as his bringing one million people to participate in the Special Administrative Region which Suzaku supports. Then, in typical Lelouch fashion, he has the million people dress as Zero, meaning they get exiled too. The entire plan hinges upon Suzaku being Lawful Stupid and obeying their deal to the letter (though in fairness to Suzaku, it relied equally on his honest desire to avert a potential riot/massacre).
Lelouch's IQ seems to randomly drop when someone he cares about is involved. The most obvious example is when his close friend and potential girlfriend is killed, which hinges on none of the important members of the Black Knight finding out but they do. He then follows it up with including an insane male Tyke Bomb who wants to be Lelouch's brother... in the team tasked with saving Nunally. Despite knowing that he killed his almost girlfriend. That's not a typo, he really did something that stupid.
After taking over the school during the finale of the first season Lelouch didn't make sure the students -including his own sister- were guarded 100% of the time. After most of the students leave Nunnally alone while trying to help Suzaku, V.V just waltzes on in and picks Nunners up. This makes Lelouch leave the Black Knights and causes the failure of the rebellion, leading to the capture of nearly the entire rebel force.
Lelouch joking to Euphemia about being able to make her murder all the Japanese. Given the way the sequence is Dude, Not Funny!, awkwardly drawn out, and begging the question "why are you saying all this?", it was rather obviously just a setup for an incoming plot twist.
The reveal that Lelouch has been just outside the base of V.V the whole time looks impressive until Fridge Logic sets in, and you figure out that V.V is too stupid to have such sophisticated things as a lookout in case an army was about to attack his base. This makes him stupider than Meerkats who have developed such sophisticated methods.
Lelouch probably ordered the guards to be taken out off-screen though.
Suzaku's entire plot throughout the series is driven by his seeming to have swallowed a single-issue-focused one of these; he apparently got the notion somewhere that a foreign-born soldier has the potential to change a corrupt system from within in a highly racist hereditary absolute monarchy, a kind of system where, historically, even high-ranking highborn nobles have little influence on actual major policy decisions of the kind Suzaku wants to change. Has the boy never taken a civics course?
Nearly Oghi's entire arc once he falls for Villetta, from letting his guard down around her during the Black Rebellion, leading to him getting shot and Ashford falling out of the B Ks control, going AWOL later on to meet her knowing full well she will kill him, all the way up to him buying into Schneizel's deceptions and turning on Lelouch. The Black Knights follow suit on the latter, and deserve another mention for not questioning Villetta, even though she's a Britannian agent who had they done so, they would have learned had been antagonizing Lelouch, and by proxy, them.
A minor one by Jeremiah Gottwald, when he makes his appearance in R2. He's just charged in on Ashford, slaughtering people who simply got in his way. He managed to defeat Sayoko by using his Geass Canceller on Rolo's Geass. Viletta shows up and tells him where Lelouch is. He knows that Rolo has aided and abetted Zero, and is thus a traitor to the Empire. He has just cut down three men for far less, so he... lets Rolo live, and just walks away from the boy who can fully paralyse people with his mind.
An episode near the end of Slayers NEXT put the plot on hold just to show a fight sequence with a Brainwashed and Crazy Gourry. Which wouldn't have been so bad if the villain hadn't slapped a really bad helmet on Gourry (and disguised him with nothing else) and also required the cast to A) Not see through the paper thin disguise B) Apparently forget that Gourry was kidnapped one episode ago C) Not assume that the blond guy dressed like Gourry and wielding the Sword of Light and fighting exactly like Gourry actually was Gourry. The characters would even constantly point out that he seemed familiar, but amazingly, despite the fact this is Slayers, they seemed to lose their Genre Savvy natures for the duration of the episode and the viewer was supposed to take it all seriously.
Many people in the Slayers universe in general tend to juggle the idiot ball quite profusely around the protagonist. In a land where Lina Inverse is not only known as a skilled sorceress but actually feared by bandits because she's known to hunt them down, incinerate them with magical fire, and then steal their loot, she seems to run into no shortage of loud-mouthed buffoons willing to comment on hersmall breasts, make fun of her height, or refer to her by one of several unflattering nicknames.
Interestingly, in one volume of the Bobobo-bo Bo-bobo manga Poppa Rocks/Don Patch uses an actual "Idiot Ball" as a weapon. Whoever is hit by it would "get an 'A' on his exam... by CHEATING!"
Don Patch also has an attack where he injects you with a serum that makes you think like he does. Beauty didn't take it well.
Light: If this phone rings, I'm dead! (beat) Wait, what am I thinking; I can just turn it off.
One of Light's few weaknesses is that, if something catches him without a plan, he's prone to panic. You can see it at the end too. But this trope is played straight with Naomi Misora, who, trusts a kid who keeps following her around telling her not to talk to the police and demanding to know her real name when all she had to do was walk three minutes down the street to the police station. In fact, if she had simply taken the three minutes out of her circuitous path to finish her walk she would have survived and Light would have been completely undone. She spiked that Idiot Ball and did a touchdown dance. Also Mello He never guesses that Matsuda isn't the Second L or thinks to search a suspect in a murder conspiracy. These mistakes ultimately cost him his life.
Light still takes the Idiot Ball to the face, however, when he stakes his endgame killing stroke on a note that he kept in an unmonitored box. It would have taken Light less than a minute to verify the deathnote would work, but instead confidently announces he's won, when just shutting up would have kept him in the clear.
In Rappi Rangai the hero has a tiny horn that caused him to be beaten and ostracized by his village as a child. This Freudian Excuse causes him to flee like mad whenever he thinks torches and pitchforks might be around the corner. Even when nothing is actually wrong. Even if it'll completely undo his Ninja Harem's plans. Even on his wedding day when his would-be bride says to his face she thinks the horn is cute. Even when the Ninja Harem tells him that his bride thought it was cute. In his defense, they were pretty severe beatings.
Tsubasa Chronicle, episode 48. After collecting 13 feathers on screen and who knows how many off screen, all with pink markings, King Chaos just hands them a number of purple, blue, and orange feathers. And not one person stops to question that. It's a trap.
Higurashi no Naku Koro ni's Keichii carries one of these towards the end of Watanagashi-hen, when he agrees to go for a walk in a secluded underground dungeon with a self-confessed murderer who freely admits to having mental issues that make it difficult to control her behavior even when she doesn't actually want to hurt anyone. She's able to hold herself together enough to let him go after only torturing him a little, but before she does she warns him never to get near her again. Guess what happens when he decides to ignore that warning.
Although, to be fair, she did confess that her sister was still alive and being held on the dungeon. Still, he could've gone with Rena or simply without Shion. Or he could've waited for the police to arrive first, that works too.
In the game version and the manga, he had a better and more justified reason for ignoring her warning — namely, he was giving Mion the doll that, by not giving it to her, started the whole thing. Still pretty dumb, though.
It seems like Momo's class in Peach Girl is forced to carry this, as it regards Sae. Her schemes have hurt or enraged almost everyone long before the first time she is caught out on her campaign against Momo. Yet the class is ready to lynch Momo, and on more than one occasion, based on setups they all know Sae's capable of, and they all say very clearly that they know this when she is exposed. Worse, even when exposed, Sae getting anything stronger than the cold shoulder seems out of the question, since no one wants to do to her deservedly what they did to Momo based on whipped up frenzy. Momo's first boy friend actually believed Sae that it was ok to kiss her as a way to practice for Momo.
In Bleach, Urahara has the Idiot Ball when he knows about Aizen's plan to kidnap Orihime and use her powers on the Hogyoku and wounded Arrancar, but instead tells Orihime that she can't fight because she's unable to use Tsubaki. Orihime then, believing that she's not strong enough to fight, goes with Rukia to the Soul Society, and on her way back, gets kidnapped. At no point does Urahara attempt to confirm her whereabouts or keep her out of the fight. What he should have done was keep her with him at his shop where, if any of Aizen's flunkies wanted to get at her, they would have to first contend with him, Yoruichi, Tessai, Ururu, Jinta and Renji.
Zommari Leroux. He could have easily ended his fight with Byakuya immediately, but was too arrogant to finish him off quickly. Lampshaded in the battle, Byakuya bluntly tells him how much of an idiot he's being to reveal his powers to quickly and freely.
Kaname Tousen. During his fight with Zaraki, he wasted time lecturing him even though Zaraki couldn't hear him. Later, he's toying with Hisagi and Komamura with his hollow mask alone. Yet he decides to use his resurrección, apparently without having tested before hand. Once he uses it he gets so intoxicated by his newly gained sight that he fails to notice Hisagi sneaking up behind him, and gets stabbed in the head as a result.
Heck, Ichigo challenging Ulquiorra to a second fight at all. The guy almost killed you in less than a minute without even trying, your best attack did almost nothing to him, it's only been two hours since your last fight, you have done zero training since then, his inferiors kicked your ass, and you not only charge in for a rematch, you actually expect to win? Wanting to rescue Orihime is one thing, but that's not trying to rescue her, that's tantamount Suicide By Espada; rescuing her would involve grabbing her and getting the hell out of Dodge, not trying to fight the guy guarding her.
That entire fight was made of stupid. Besides the above examples, Ichigo telling her not to "interfere" again, Orihime crying over Ichigo's dead body instead of trying to help Uryu, Ulquiorra not grabbing Orihime and flying away when the fight turned sour...seriously, the only one not holding an Idiot Ball was Uryu.
And back to Ichigo one more time as after he finds that his Super-Powered Evil Side has shredded it's way through Ulquiorra he begins freaking out about not wanting the fight to go that way. So on the face of it we get to see him placing honor in front of the whole rescue mission they're risking their lives to do but even if you accept that what's NOT acceptable is Ichigo's immediate demand that Ulquiorra start chopping off some of his limbs so the fight can be fair again. The final nail in how ridiculous that is is that Ulquiorra went to great lengths to explain that he has regeneration powers and can explicitly restore lost limbs, if Ichigo really wanted to make things fair again all he would have to do is stop and let Ulquiorra naturally heal.
The most egregious example of idiot ballage is perhaps when they break into the enemy headquarters full off some of the most powerful enemies they've had to that point and decide to split up. Apparently only because of a few individuals' sense of pride — it's kind of sad when Ichigo the resident reckless hero, is actually the voice of reason that gets summarily ignored.
The Vandenreich, initially introduced as Dangerously Genre Savvy opponents who know not to screw around, end up juggling it as their initial invasion of Soul Society progressed. At first they slaughter their way through the Shinigami without saying a word, but eventually start chatting them up (which even their commander and second-in-command acknowledge is a problem.) In fact, two of them are Killed Mid-Sentence while explaining their powers. The one who stole Chojiro's bankai thinks that having it means that Yamamoto is no match for him (Yamamoto being the guy who beat their supreme leader in the past) and three of them jump Yamamoto right when he's fighting their leader, with them ending up fried by his zanpakto. The last one is particularly bad because one of the guys who did it was As Nodt, who managed to overcome Byakuya and decided to take Yamamoto bare handed despite having at least three different ranged abilities he could have used. Yamamototakes out all three instantly with just a brief glance away from his enemy.
It's subverted with the implications that the jump attack may have been planned beforehand, as Bazz-B boasts about how he offset Yamamoto's flames with his own.
The Vandenreich's second attack has been more prone to this trope than the first. A rather blatant case with Cang Du, who up until then had not said a single word and been presented as The Stoic. In this invasion, the first thing he does when confronting Hitsugaya is explain his personal beliefs, giving Histugaya the opportunity to get back his bankai. Seriously, what was he thinking? Especially considering Bazz-B had almost killed Hitsugaya beforehand.
The usually Genre Savvy Kyoraku gets hit with a surprisingly bad example of this, when, as the newly-instated Captain-Commander of the Gotei 13, he has Unohana partake in a Duel to the Death with Kenpachi so that the latter can get stronger. The Gotei 13 has about 3000 members or so, approximately 200 to a Squad; at the end of the Vandenreich's initial attack on the Soul Society, the number of casualties is recorded to be over 2500. What's worse, the Royal Guard, who might stand a chance against this new threat, is established to only care about protecting the Soul King and the rest of the Royal Family (and not the Seireitei), as evidenced by their arrival after Vandenreich butchered the Gotei 13 (though they didhelp train Ichigo, Rukia, Renji, and Byakuya after healing their injuries). As such, Unohana ends up dying in mortal combat after getting Kenpachi to stop subconsciously limiting his power, so instead of the Gotei 13 having both Unohana—their best healer and second-strongest combatant (after Yamamoto) going into her duel with Kenpachi—and Kenpachi (who could have been brought up to his current level without Unohana dying), they're short one very versatile and effective Combat Medic. (Shinji even notes in 581—as four Sternritters invokeWorf Had the Flu and attempt to kill a heavily worn-down Kenpachi, whom Shinji says is the only character capable of putting up a fight at the moment—that while Momo is a good enough healer, she's nowhere close to Unohana's level of expertise.) It's a good thing for them that Ichigo arrived in the nick of time to bail out Kenpachi because if things did go south during the Vandenreich's current assault (which could still happen now that Yhwach has infiltrated the Royal Palace), the Soul Society would be done for.
Bleach's Filler arcs have a lot of characters holding the Idiot Ball, but Big Bad Muramasa from the Zanpakutou Tales Arc deserves a special mention. Early on, Byakuya Kuchiki apparently defects to the Zanpakutou's side. The only reason he gives is that he's protecting his pride - something that no one in the team actually believes in (everyone thinks he must have some sort of plan). While Muramasa does suspect that Byakuya's motives aren't really those he states, he still lets him tag along to the Real World, when he is at his weakest, to help him release his master, Kouga Kuchiki. What do you know, turns out Byakuya is a Fake Defector, whose real reason to follow Muramasa is to have an opportunity of killing Kouga as the current leader of the Kuchiki Clan, and he goes on to do just that. Okay, so Kouga Kuchiki is a Jerkass who, upon awakening, tries to kill his Zanpakutou... Still, one can't help but wonder what would be Muramasa's reaction when he realized he indirectly caused his master's death.
So, Lawrence. You've got a harvest goddess as a traveling companion, who prides herself at being "Horo the Wise" and being far cleverer than you give her credit for. You've already been called out for keeping secrets from her, and you had previously flubbed your Insight check to pick up on her mood when you suggested you just drop her off at the nearest town instead of going all the way to Yoitsu with her. Was it really such a good idea to leave her with the letter that says her hometown is probably gone, just because you assume she can't read?
He didn't just assume - she told him she couldn't read. And she did it probably so she could play some kind of trick on him, while he kept a secret to spare her feelings. Their fight was a result of mutual miscommunication.
The entire storyline of Angel Densetsu juggles with so many idiot balls that you have more or less every example, subset or relation of this trope. In spades. We even have Word of God that Kuroda is the designated carrier and master of the Idiot Ball, for the express purpose of plot advancement. Behold the Idiot Ball of Plot Advancement! The worst thing? It's so well done it's almost believable.
There's so many balls zipping around it even takes a while to understand who the Butt Monkey is. (It's Ogisu)
Greed of Fullmetal Alchemist has the power to make himself invulnerable in a way that can only be circumvented by five people in the world. He rarely extends the power beyond his arms; he doesn't like covering his whole body, because he thinks it makes him look ugly.
He also can't regenerate and use armor at the same time, as Ed notices during his fight after using a transmutation technique that makes Greed's carbon armor useless. King Bradley/Wrath doesn't give him time to do either during their first fight.
In the anime, Greed goes full-armor when it's serious business. Any other time, he's still armored below the skin, as Izumi can attest to. "So, how many fingers did you break when you hit me just now?"
In Hajime No Ippo, Kobashi, a physically weak boxer, uses a boxing style that goes for a win via points and exhausting his opponent instead of going for a knockout. However, during the last round of his match against Ippo, Kobashi, having a two point lead and an opponent too exhausted to catch him (meaning that all he has to do to win is dance around until time runs out), suddenly decides to try to go for a knockout against an opponent known for KOing all of his opponents. This idea ends about how you'd expect it to. This is made even worse by the fact that Kobashi had apparently put a great deal of research into Ippo's fighting style before the match. Kobashi even lampshades it after he wakes up.
It actually makes more sense in context. Kenta fights the way he does because he doesn't have the muscle to win by KO, and instead plays to his advantages. During the end of his fight with Ippo, when he has Ippo at his mercy, he lands a really good hit on him and is blown away by how good it feels and pushes to get a KO.
In Yu-Gi-Oh!, Joey duels Kaiba for the first time in Duelist Kingdom. After Kaiba plays his Rabid Horseman on around the second or third turn, Joey throws out several weak monsters in attack mode, despite the fact that there's a defense mode that is used in almost every duel before and after this. It's unknown why Joey wouldn't use it, besides perhaps not knowing how the new duel disk worked. However if this is the case, he could simply have just asked Kaiba how the machine worked (this was the first duel between two living humans to ever use the technology). If Kaiba didn't oblige, this would more or less be cheating.
After Kaiba played Horseman the first monster Joey sent after it was Swamp Battleguard a monster with 1800 attack points and after it was destroyed Joey sent Axe Raider with 1700 points to avenge Battleguard, yeah he tried to send a monster that was weaker that the first one thinking it would do better. And all of the other monsters Joey attack R.H. with also have 1800 attack points. Joey, if the first 1800 attacker couldn't beat Kaiba's monster what makes you think the others will? It's no wonder Kaiba insulted him to no tomorrow over it.
In the beginning of the second season, Yugi, who was on the way to school with Tea, was naive enough to lend the fortune teller his Millenium Puzzle in order to get his fortune told, and he would have ended up dying in a warehouse fire if it weren't for Joey and Tristan.
Arguably, his attempt at a Heroic Sacrifice later in the same season during his forced duel against Joey as a part of Marik's plan could be seen as this. Yugi has just absorbed Meteor of Destruction with Mystical Ref Panel, and decides to nearly kill himself with it so Joey could be free, but Joey tearfully calls him out on this and gets Red-Eyes Black Dragon to attack him so that he could free Yugi.
On the same series, we have the infamous ending in Yu-Gi-Oh! 5Ds of the WRGP duel between Team 5Ds and Team Unicorn. Both teams are down to their final duelists, Yusei and Jean respectively. It's Jean's turn, and Yusei's deck is completely empty. Simply doing nothing and passing his turn would cause Yusei an automatic loss, shown Team 5D's (who, due to the double-loss elimination rule for the prelims, would not have been out of the competition) a great lesson in the effectiveness of a team building their decks to work together as opposed to the haphazard strategies theirs possessed, and given them all a great bit of character development....instead, Jean goes for the win by attacking one of Yusei's Defense Position monsters, who then gets a massive enough Defense Point boost that the backlash damage causes Jean to lose. Why does he attack? Because Yusei opened his eyes to how fun Dueling could be, lodging the Idiot Ball firmly in his chest.
Given how often these Duels become life-and-death situations, it's amazing this wasn't a case of Too Dumb to Live.
In Chapter 79 of Soul Eater, Tezca Tlipoca, known to be Shibusen's best tracker, walks blindly into Noah's base, not taking any apparent precautions for the event of traps. Sure enough, he gets hit with a powerful attack from underground.
Chapter 82: Guess what? He's not dead. He had someone else go and die instead of him. No idiot ball on Tlipoca. Until he uses a gopher mask as a disguise anyway.
Extra idiot ball time since he shuts them down and locks them into stasis because the eternal energy supplies make them so uncontrollable then when he rebuilds himself into a Cyborg like them uses the older inferior energy absorption design even though logically he's not going to be worrying about being unable to control HIMSELF.
Screw that. He could make bombs strong enough to kill Perfect Cell and tiny spy robots that nobody noticed by years. He could've combined both into an Action Bomb and kill Goku that way.
Turns out, the bomb's strength may have been an overestimation. And he was probably using it as a contingency plan if it turned out Goku is still stronger than his creation...he just didn't see his creations ever turning good, and someone removing the bomb.
Goku, Bulma, Yamcha, Oolong, and Puar all cling to the ball when they invade Pilaf's castle. They come across a series of arrows painted on the floor and all decide to follow them. Not surprising, they lead to a dead-end and a wall drops behind them, trapping the group. It then cuts to Pilaf and Mai watching the group on a monitor.
Piccolo gets tossed one in the Cell Saga when he forgets that Cell can sense energy, and that fighting Android 17 at full power would bring Cell running. This is especially painful to watch given that Piccolo is normally one of if not themost intelligent characters in the entire series.
Piccolo probably did that because he failed to anticipate that Cell could grow strong enough to outclass him and 17 on a diet of normal humans. This is no excuse, as he did the same mistake with Frieza (except that guy only had transformations, unlike Cell) and paid for it with what was until then the ass-beating of his life.
Ciel Phantomhive is thrown the idiot ball in the second season of the Kuroshitsuji anime, when he doesn't question why Soma knows him and Ciel himself doesn't remember him. Instead of wondering about this also, Sebastian answers with "Foreigners are difficult to understand."
Kyouko got more of an Optimism Ball that worked too well. She believes Kyuubey that Sayaka could be turned back from a witch, despite the fact that Kyuubey admitted to setting up the whole thing. She doesn't ask Homura, who has known about the whole system from the start, if he's withholding information. Finally, she brings Madoka (who is completely defenseless) to the fight, while Kyouko herself has a dimmed Soul Gem from overuse of magic. It ends with a Suicide Attack, per Kyuubey's Batman Gambit.
There's also the scene where Madoka throws Sayaka's Soul Gem off a bridge. Not the smartest move, though it can be a little understandable since the girl was quite the emotional wreck already... but even worse is how Homura is nearby, knows from experience how bad that sort of thing can turn out, and makes no real effort to stop it until it's too late. What,did she not have enough time?
She may have allowed this to happen on purpose, so that Madoka would learn how literal the name "Soul Gem" is, and would be less likely to want to become a magical girl.
In one episode of The Big O, Roger chased Allen Gabriel onto an abandoned train in an empty train station, despite knowing beforehand that he himself is unarmed and that Allen Gabriel loves to kill people. Angel even lampshades what a dumb move this is.
In Magical Girl Lyrical Nanoha, Lindy gripped the idiot ball pretty hard when she sent mooks to try to arrest Precia, the most powerful mage in the entire series, who had already demonstrated that she was capable of magic powerful enough to disable a spaceship from another dimension. The results are predictable.
Nove gets one in StrikerS when she lets Teana know that the Numbers are listening in on her telepathic conversation with Subaru, making her aware that her plans had been compromised and preventing her from divulging any more information.
The Unknown Enemy from Gundam AGE has an egregious cases of Idiot Ball holding in the case of Desil Galette. They could've trained the seven-year-old boy, who's a powerful X-Rounder and has lots of potential as a fighter, to make him a better pilot and control his nasty streak. Not only they don't, but they give the kid a commanding position! As a result the kid is so unruly and spoiled that he calls out a retreat from a certain victory because he got bored, and when a second X-Rounder is recruited... she's put under Desil's command instead of having her trained as well, and she ends up killed by Desil himself.
However, we later learn that he gave the wrong answer because he screwed up one number, proving his innocence of the whole thing (before that point, he was framed by the villain). This doesn't fix the whole world ending-thing, but still.
Cytomander suffers a terrible case of the ball in Gurren Lagann- brought on by a fit of insanity in utter disbelief of the Holy Shit Quotient that has befallen his entire fleet. And it ends horribly... as in not seeing a colossal warship barreling right down his throat until it plows him into his own ship and makes a fried chicken.
In Thunderbirds 2086 (not the puppet version), vetted astronaut and spaceship pilot, Captain Kallan James, in the Episode 6 "Sunburn" actually asks "What's a ramjet?"
The first episode. Okay, so the protagonist is a top-ranked player in this online game where you try to blow up your opponents with bombs. Then he suddenly finds himself on an island with a bag of things that look just like the bombs in the game, but he doesn't recognize them; he picks one up, presses a button on it, and a timer starts counting down. He still doesn't get it, but he "has a bad feeling" and throws the thing away just before it explodes.
He now realizes that they are bombs, but he still doesn't know how to use them. After letting a random bomb-throwing psycho chase him halfway around the island, he decides to fight back by throwing a bomb...without pressing the button. After the bomb fails to explode, he has the Shocking Realization (complete with Viewers Are Goldfish flashback to three minutes ago) that you have to press the button first! So he tries it that way, and it goes off too late. After a few minutes of screwing around incompetently, he has another Shocking Realization and flashback revealing that the numbers counting down were probably a timer! Eventually, he drops the Ball by tricking the nutjob chasing him into standing on top of a bomb. About time, Mr. Expert Gamer.
Claymore: Teresa is antagonized by a group of bandits who know she's extremely powerful (they just don't realize how far the "extremely" goes with her) but strictly forbidden from taking human lives, but lets them go free and (almost) completely undamaged, until they, being exactly the type of cruel assholes one would expect bandits to be, eventually provoke her into breaking that rule by torturing her protege. As far as anyone knows, Ain't No Rule about incapacitating them nonlethally, and bringing in humans who are willing to kill to protect their families from bandits to do what she isn't allowed to. Later on, when her erstwhile comrades are hunting her, she decides to kill Priscilla because she has the potential to challenge her in the future, and fails to do so several times despite every opportunity.
Valvrave the Liberator: Shouko and the others hand Haruto and L-Elf over to the forces of Arus and Dorssia, in exchange for being allowed to leave after they had killed just about most of the student body. Not only did this not work, but the moment they got their hands on them they prepared to blast them away and it took a Heroic Sacrifice to let them escape.
A lot of problems in the second half of Doki Doki Pretty Cure would have been solved had Mana not decided right then and there to not use her super special purification special attack on her friend who just got Brainwashed and Crazy. She ends up with multiple chances to do so, but she (or the others) doesn't take it, instead trying I Know You Are In There Somewhere Fight speeches (or in Makoto's case, song).
The World God Only Knows: In the last bit of the Goddess Arc. Keima has to choose between pursuing Ayumi and Chihiro. Chihiro has confessed her love, but not to remembering her conquest. Ayumi hears said confession, and goes from being tsundere to kindly and supportive. Keima tries for Chihiro first even though his normal game rhetoric would tend to proclaim the obvious choice was the wrong one. More than that, three of the previous goddesses had strong thematic links with their hosts (Apollo goddess of music and art is in idol singer Kanon, Mars goddess of war is in boisterous, aggressive Yui, and Minerva goddess of wisdom is in bookworm Shiori). Merucy/Mercurius is the fleet footed messenger goddess, and Ayumi is the track star.
The Island arc in Assassination Classroom is centered aroud Class E's assassination attempt on the titular vacation island. Everyone has lots of fun during the day, including Koro-Sensei, who gets so sunburned he becomes charred black. Fortunately, he has a "molt the skin" technique that can be used once per month and is usually used to leave a decoy. Too bad it uses quite a lot of energy, enough to slow him down... and that Koro-Sensei promised to let the students prepare an assassination attempt that very night... and that he also promised to let them shoot 7 tentacles, which, just by itself, reduces his speed to a 1/16th of his normal speed of Mach 20.
A rather frustrating example occurs in the Death 13 mini-arc of Part 3 of Jojos Bizarre Adventure, where everyone carries the Idiot Ball. Everyone except Kakyoin refuses to believe that a baby (the villain of that particular mini-arc) could be a stand user (when they've already encountered a stand-using orangutan, so a stand-using baby shouldn't be that hard to believe) while Kakyoin (normally one of the calmer members of the group) resorts to hysterics to try and convince everyone. The frustrating part comes from the fact that they're normally Genre Savvybordering on paranoid, immediately assuming that anything unusual happening is the result of an enemy stand.