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).
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.
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.
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.
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.
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.
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.
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.
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 paralyze people with his mind. And this one moment caused almost all of the idiot ball moments listed above.
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 intelligent 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. Outside of this particular instance, one of Light's few weaknesses is that, if something catches him without a plan, he's prone to panic. It's often internal, but not always. You can see it at the end, too.
Naomi Misora trusts a kid that simply tells her that his father is the leader of the Kira investigations at the headquarter and says she should tell him everything, which he'll then pass onto his father. Instead of simply trying to talk to a detective at the police station at some other point, she tells Light all of her information and theories about Kira, causing Light to panic cause she's spot on. Alternatively, she was already smart enough to give a fake name, and would've beaten Light if not for Ryuk's entirely unsubtle laughter giving him an unfair advantage. She could have just given another, and any suspicion he may further have that it's fake would've cast doubts on who he was, especially since she knew to trust no one but L. Instead, she gave her real name, spiked that Idiot Ball and did a touchdown dance.
This is especially jarring if you happen to know about her role in Another Note. It's like, "Did you learn nothing from your experience with Beyond Birthday?!?!?!"
Mello never guesses that Matsuda isn't the Second L, or is related to that team, 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.
Also played straight when Matt loses focus while he's supposed to be spying on Misa, allowing her and Mogi to make an escape.
Mello removes his helmet in front of Takada and allows her to cover up with a blanket when he makes her remove her clothes, which allows her the opportunity to write his real name down on a piece of the Death Note she had stored in her brassiere.
Matt attempts to talk his way out of a confrontation with several of Takada's bodyguards, all of them armed. In doing so, he decides to be a wiseguy and crack some racist dick jokes...and then says You Wouldn't Shoot Me. Guess what happens.
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 Reservoir 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: When They Cry'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.
Urahara knows Orihime's powers will interest Aizen enough to kidnap her, but only tells Orihime to stay off the battlefield because she's unable to use Tsubaki. Rukia take Orihime to Soul Society to make Orihime a stronger fighter, but Orihime is kidnapped on her way back home. At no point does Urahara keep tabs on Orihime and, as an expert in breaking into the allegedly unbreakable Dangai, he'll know that location is as vulnerable to attack as any other. She would have been safer remaining at the Urahara Shop where she'd have to contend with him, Yoruichi, Tessai, Ururu, Jinta and Renji.
Zommari could have ended his fight with Byakuya immediately by going for a headshot before Byakuya knew what his power could do. Instead, Zommari used his own arrogance to try and bring down Byakuya's arrogance. Lampshaded during the battle when Byakuya bluntly tells him how much of an idiot he's been to reveal his powers too quickly and freely.
Tousen wasted time lecturing Zaraki during their fight, despite Zaraki being incapable of hearing him. It gives Zaraki time to adapt to Tousen's Bankai and defeat him. When Tousen later fights Hisagi and Komamura, he decides to use Resurrección without testing it before hand. He had been toying with his opponents previously, but after the power up becomes so intoxicated by his newly-gained sight that he drops his guard and gets ambushed by Hisagi. Hisagi lampshades this at the end of the battle.
The Vandenreich are initially introduced as ruthless opponents who take advantage of the Shinigami habit of explaining their powers. As they become more exposed to the Shinigami, they become more talkative and also start explaining their powers. The Vandenreich leader and his second-in-command discuss this being a problem, but two get Killed Mid-Sentence and the one who steals Chojiro's Bankai thinks his success makes him a match for Yamamoto. He's wrong. As Nodt is introduced as particularly calculating and ruthless to defeat Byakuya, but is later seen joining two other Sternritter in ambushing Yamamoto. It forces Yamamoto to look away from his opponent to deal with them, but that's all it does.
Cang Du is mostly a silent character until he confronts Hitsugaya. He explains his personal beliefs, which buys enough time for Hitsugaya to benefit from Urahara's intervention to restore his Bankai. Bazz-B almost killed Hitsugaya, but Cang Du wastes it. Lampshaded in-universe by both Bazz-B and Yhwach himself.
Kyoraku's decision to make Zaraki more useful in battle against the Sternritter results in getting a Shinigami killed, something he knew in advance would happen and carried through anyway. His first act as Captain-Commander is to order Unohana to power up Zaraki. The Gotei 13 loses just over 2500 out of 6000 men in the first Vandenreich invasion and the Royal Guard prioritises the Soul King over helping out. Even though Yamamoto never let her fight, and Zaraki does get powered up (but not enough, it turns out), it costs Unohana her life and the Gotei 13 its best healer.
Muramasa from the Zanpakutou Tales Arc accepts the reason for Byakuya's defection to his side as "protecting his pride". While Muramasa is suspicious of Byakuya's true motives, he still lets Byakuya tag along with him when he attempts to release his master, Kouga. Kouga is a disgraced member of the Kuchiki clan whom Byakuya needs to locate in order to kill him and remove the stain on the family's honour, something he couldn't do without Muramasa's help.
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.
Jonouchi duels Kaiba for the first time in Duelist Kingdom. After Kaiba plays his Rabid Horseman on around the second or third turn, Jonouchi 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 Jonouchi 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 Jonouchi sent after it was Swamp Battleguard a monster with 1800 attack points and after it was destroyed Jonouchi 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 Jonouchi attack R.H. with also have 1800 attack points. Jonouchi, 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 anime's second season, Yugi, who was on the way to school with Anzu, 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 Jonouchi and Honda. This is different from the manga, where the Puzzle was taken from him during the Dungeon Dice Monsters arc.
Arguably, his attempt at a Heroic Sacrifice later in the same season during his forced duel against Jonouchi 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 Jonouchi could be free, but Jonouchi tearfully calls him out on this and gets Red-Eyes Black Dragon to attack him so that he could free Yugi.
Yami Marik's victories over Mai and Yami Bakura were achieved thanks to their fatal misplays. Mai had the opportunity to defeat him with her Harpie Lady Sisters, but she decided to tribute them to summon Ra... which she couldn't control and it remained in its Sphere Mode until Yami Marik unleashed it and defeated her. While Yami Bakura could control Ra, Ra had 0 ATK, but 3450 DEF and is very hard to defeat since it is also immune to most effects. Despite this, Yami Bakura tributed his Stone Wall for his Dark Ruler Ha Des, forgetting that he "traded" his Spell Card Monster Reborn during the duel, which gave Yami Marik the opportunity to get Ra back. Then again, Yami Bakura assumed that it wouldn't be a problem, since Ra's default attack power is dependent on the attack power of the monsters tributed to summon it, meaning that its attack power would be 0 if it were special summoned. Too bad only Yami Marik knew of Ra's secret ability that allows the player to convert their Life Points into Ra's Attack Points. On one hand, to be fair to the two of them, Ra's highly mysterious nature means neither of them actually knew how the card worked until it was too late, but even then it still counts, since trying to use a card you know nothing about when you have a much simpler method of victory in hand seems like a totally unnecessary gamble, especially for Yami Bakura, who should've realized there was more to Ra than met the eye.
On the same series, we have the infamous ending in Yu-Gi-Oh! 5D's 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.
Dr. Gero, lead robotics scientist of the Red Ribbon Army, shuts 17 and 18 down and locks them in stasis because their eternal energy supplies make them too powerful to be trusted not to turn against him. Then when he rebuilds himself into a cyborg like them, he uses the older inferior energy absorption design... in case he turns against himself?
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.
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."
Puella Magi Madoka Magica: Kyoko 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.
Puella Magi Madoka Magica: Rebellion reveals that Homura made a big one in the finale of the main series, telling Kyubey about Madoka and her wish allowed him to kick-start the plot of the movie.
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 Magical Girl Lyrical Nanoha 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 Mobile Suit 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 Tengen Toppa 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.
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 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.
Chairman/Principal Asano has grabbed this and the Villain Ball hard in regards to his treatment of Class E. Beating down the confidence of the only people in a legitimate position to end a world ending threat that will destroy ALL LIFE on the planet within a year is a terrible idea. If they fail due to, say a lack of confidence in their own abilities thanks to a systematic destruction, then he's equally culpable for the destruction of the human race in order to preserve his ideals as a teacher which favors grades over individuals. Worse, if they succeed? Then a phenomenally rich, highest quality assassin who has the favor of every government on the planet, the assistance of several of the worlds best assassins and an overtly obvious reason to HATE the principal for making their junior highschool years a living hell, has every reason to ruin/kill him with every resource available to them and basically no one to tell them not to. Anyone with real authority knows they saved the world, right?
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 Properly Paranoid, immediately assuming that anything unusual happening is the result of an enemy stand.
In Fairy Tail, Team Natsu (plus Lily) charging intoHades' ship while exhausted and with the knowledge that Hades defeated Makarov. They didn't even need to since Hades wasn't going anywhere and his objective (Zeref) was elsewhere. Grabbing Zeref (Gray even knew where he was) and leaving would have been infinitely more intelligent and was never even considered. This is more an example of Honor Before Reason. Perhaps Natsu wouldn't have considered running with Zeref under any circumstances, but he'd be the only one of that group. No, Grimoire Heart had kicked in their door, basically invaded their home and trying to kill them all off, on top of successfully destroying the island. It wasn't that they were unable to consider tactics, it was that they were pissed.
Kagewani has Takeru still trying to get cryptid footage for his documentary despite the fact that his crew were killed in the forest by said cryptid.