Talismans of Shannara does this to Wren, who just spent the last entire book learning how easy it is for trust to be betrayed, but who still falls for the trap of a character everyone (including her) suspected, just to get her kidnapped and in contact with the other good guys to tell them what she knows. When she gets back, nothing horrible seems to have happened to her army in her absence.
Grace in Vampirates takes forever to figure out that she's on a ship crewed by the titular creatures, despite knowing that they exist before they find her.
In the fourth Magic Kingdom of Landover novel, Ben Holiday is informed by his long-time nemesis Nightshade that his newborn daughter will be the target of her next scam. In the fifth novel, she goes through with this with a very obvious scam, and the one person who Ben refuses to believe could possibly be behind it... is Nightshade. He then spends the entire book stalwartly seeking the help of everybody in the world to identify who the real threat is and help him overcome it, only realizing that it's exactly who he should have known it was all along when he's told as much by his other long-time nemesis, Strabo.
The Draka series of Alternate History novels, in which enemies of the Dominion outright ignore it until it's too late. Repeatedly. Knowing it's happened before. As one synopsis put it, the first 'alternate' in the setting's history must have been an 11th Commandment reading "Thou Shalt Not Attack the Dominion of Draka".
How much of Europe had Hitler taken before the Allies finally did something? All through history you will see people saying "Maybe if we just ignore it it will go away!"
This is actually directly addressed in the novels. The Europeans of the time viewed the Domination as a barbarian (nonwhite) nation with a thin veneer of civilized (white) rulers, certainly not a threat to the masters of the universe in Paris, Berlin, Rome, etc. And hey, who had they conquered up to that point? Some spear-chuckers in Africa, ragheads in the mideast, buncha Chinese out in the Orient, and the Turks? Nobody important or formidable, right? Racist and stupid, sure, but that's not even remotely unusual. Even in the real world, the Nazis and the Japanese military had convinced themselves that a 'mongrel' country like the USA could never be a real threat, despite the reality of the economic, industrial, and military situation.
Interestingly enough, the Draka themselves begin to fall victim to this same attitude, referring to the Alliance as "feral humans" even before their genetic engineering programs really got going. They still won the Final War, but it's clear they weren't expecting to take the casualties they did in the process.
This Alternate History page actually points out all the historical problems the canon timeline has and how illogical the entire idea is.
For the Europeans to view the Draka as harmless requires them to ignore an empire which controls all of Africa and the Middle East, has the world's largest GDP, possesses the most technologically advanced military in the world (as demonstrated in multiple wars Europeans were involved in), and displays a tendency to kill hundreds of thousands of revolting slaves. Ignoring an entity like that when it's just across the Mediterranean is flat-out insane.
Herodotus tells of a king named Croesus who consults an oracle to determine whether or not he should attack an enemy's kingdom. The oracle says that if he attacks, "a great kingdom will be destroyed," at which point Croesus commits his army and ends up losing his capital city and being enslaved. He could've just asked which kingdom would be destroyed, but that's not nearly as fun, is it?
The Oracle was always intentionally vague, and there was even a quote upon the entrance to her temple saying, in effect, "Know thyself." In that respect Croesus held the idiot ball in that he overestimated his forces.
Also, asking just one question to the Oracle was a huge deal, as the questioner was expected to make an offering to the Oracle before asking. Croesus brought a HUGE load of loot all the way to Delphi, so asking "Which Empire?" would mean a trip back to Lydia, emptying more of his treasury, then another trip back to Delphi. Not the most economical way of divination, or maybe Croesus was just stingy.
To be fair, when the Persian King Cyrus allows him to send a message to the Oracle and he asks why he was told this, the Oracle tells him he should have asked what was meant and Croesus agrees.
In Eldest, two of the Big Bad's henchmen show up in the hero's hometown looking for the hero's cousin. The two characters in question are instantly recognized as the monsters who killed the hero's uncle by the townspeople, who tell the henchmen that the hero's cousin is out hunting and they don't know when he'll be back. Let's repeat that: the townspeople, in order to protect the hero's cousin, tell two known arsonists/murderers that he's out and will be back later, apparently expecting the henchmen to turn around and go home.
Mara Jade didn't even find it a little suspicious that she couldn't sense Nom Anor with the force. If she'd confronted him on it right off the bat, she'd have prevented about half the problems in the entire New Jedi Order series. Way to go, Jade.
Nom Anor grabs it too. After ten years of spying on them, he doesn't even think it's a little dangerous to try and kill Luke's wife.
In the X-Wing Series, Corran Horn is an ex-cop turned fighter pilot for the Rebellion. He's given a cover identity and put on CoruscantImperial Center, the capital city-world of the Empire, to gather information as part of an operation to take the planet. But one day, after one of his companions tries to have sex with him and he refuses, he decides to go walking while ruminating on his past, and pays no attention to where he is going. At all. He finds himself at a Wretched Hive and as it turns out the whole thing acted as a Shaggy Search Technique, but really, wandering an enemy-held world and heading into the depths of its seedy underbelly while not paying attention to where he's going? The Force looks after fools.
Corran was mostly distracted by Tycho being on the planet and dumb luck on a planet of billions (maybe trillions) that the one criminal with a grudge finds him. The real ball comes from Corran, whose 'cop instinct' just won't let him stop believing Tycho was a traitor despite the fact if he was brainwashed Isard could have used him at any time in the countless missions to take Imperial Center.
In Starfighters of Adumar, Wedge Antilles's decision to confront Tomer Darpen during a diplomatic event, letting the latter know exactly how much Wedge knew about their plans, was almost certainly a factor in the latter's attempt to get Wedge killed in combat after Wedge declined to participate in Cartann's coming war of conquest. It's possible that person would have tried it even if Wedge hadn't said anything, but Wedge really should have known better than to let a person how much he knew while said person was still in a position to do something about it. Especially since in this case, Darpen was supposed to be their ally.
Charles Todd's A False Mirror starts off with several characters playing "catch the Idiot Ball":
Stephen Mallory, confronted by local detective Bennett about the mysterious disappearance of Matthew Hamilton, husband of Mallory's former girlfriend Felicity, immediately charges off to see Felicity. This despite the fact that Mallory is automatically the prime suspect, since he is still obviously obsessed with Felicity.
In the course of evading Bennett, Mallory runs over Bennett's foot with his car. This does not make Mallory more popular with the local constabulary.
When the police catch up with Mallory at the Hamiltons' house, Felicity has the brilliant idea of pretending that Mallory is holding her and her maid, Nan, hostage. At gunpoint. It belatedly occurs to her that this is the epitome of an Idiot Ball maneuver.
To make matters worse, Nan tells two different people, including Inspector Rutledge, that Felicity set up the mock hostage situation. Nobody feels obligated to inquire further.
In "The Marching Morons" by Cyril M. Kornbluth, the population of Earth has literally "Bred for stupidity" by smart people choosing to have fewer children while idiots continue to breed indiscriminately. They have to resort to reviving a Human Popsicle to solve the problem.
Victor Frankenstein, in Mary Shelley's original Frankenstein, is warned that the monster will visit him on his wedding night. So he leaves his new bride alone while he goes outside to reconnoiter the area. And he's surprised when the monster breaks into the bedroom and kills his wife? Not only that, Frankenstein could have avoided the whole problem if, instead of "aborting" the creation of a bride for the monster because he was worried about hordes of the creatures overrunning the world, he'd simply made her infertile. (As for the last, though, it has been argued that perhaps it wasn't obvious to the writer — back in those days, their science being whatever it was — that this should have been a possibility for someone who could create an approximation of a human being out of nothing much.) To be fair, the creature often watched Frankenstein work, so even rendering the bride infertile wouldn't have stopped the creature from making more like itself. No, Frankenstein's turn with the idiot ball comes when he doesn't talk to his creation, pointing out that while the creature had promised to go away and live quietly with its new bride, neither of them could assume that the bride would agree to the same arrangement. No, he just panics and destroys the bride while the creature watches, because Poor Communication Kills.
Huge example in the Mortal Engines series: The protagonist is just about to escape from a city under siege using an air balloon, which is about to fly to the place his love interest is staying. One idiot ball later, he jumps out of the balloon, while it's taking off, while everyone's shouting at him to stay in the basket, while the city's being destroyed, and runs off to fetch a letter from said love interest. A letter he's already read. Needless to say, the balloon's gone when he gets back.
In case you are referring to Theo Ngoni in the final book, then it's more like this: A major character (not the protagonist) is about to escape from a military base which is attacked by traction cities. And it's about to be overrun. He is just about to board an airship along with his companions (which will take them not to his love interest, but simply to a safer place) when he remembers the letter and runs back to the base. His friends conclude that they are forced to leave him behind, the base being bombed and all. After finding the letter from his love interest, and also a photo of her, he runs back again, but is hit by a grenade from the attacking cities. He survives. Still a strong case of Idiot Ball.
In A Connecticut Yankee in King Arthur's Court, said King and Yankee decide to do some legwork among the peasants. What gives away their disguises? The King babbling on about how turnips grow on trees and apples below ground.
Justified, as the King is shown to have no knowledge of anything outside his castle. In fact, the whole point of this trip is to educate the king.
Everything Eladia has done from book 5 onward, assuming she was actually smarter than that to begin with. She only got worse after being corrupted.
Moiraine insisting on upholding the tradition of Aes Sedai superiority by withholding information (even from the ta'veren).
Nearly all the characters seem to have this version of the Idiot Ball. Nobody ever tells anybody anything. It's justified occasionally, but you'd think there would be a little more truthfulness between the cadre of friends meant to save the world.
Suian Sanche sending untrained girls after a group of thirteen Black sisters. She didn't trust anyone else, but that's no excuse.
Said untrained girls manage to carry the ball themselves. When Mat breaks into possibly the most heavily guarded fortress in the world to save them and rather reasonably objects to them assaulting a helpless woman they decide to handle this complaint by tossing him around with their powers. Yes, the woman was blocking them from using their powers and they needed to punch her to stop that but they never bothered to explain that, simply deciding to threaten Mat. It isn't until many books later (with the prodding of one of the few intelligent characters) that they actually give a half-hearted thanks.
A moment in the first Kingdom Keepers book. Finn and Philby, looking for clues, search Splash Mountain. Since the ride inactive, they don't take a car, facing Hazardous Water. Philby waits until they are well into the ride before telling Finn that they can't get out on the sides without setting off alarms. Meaning they have no plans to survive a four story drop.
The Eighth Doctor Adventures give the Doctor a Worthy Opponent who is about twice his size and apparently has twice his brainpower, having to explain to the Doctor the Whoniverse's second-most-obvious explanation for someone becoming a bit slimmer than they were previouslynote Turns out the most obvious explanation, diet and exercise, doesn't apply, but probably continues to plague the Doctor with confusion to this day:
An Eleventh Doctor book called "Hunter's Moon" has Rory, probably the most level-headed one between him, Eleven and Amy, gambling away the TARDIS and getting himself kidnapped to a place where humans are hunted.
Digital Fortress, where a half dozen computer scientists and mathematicians spend six pages scratching their heads before realizing that the word PRIME in a password could refer to Prime Numbers instead of Prime Ribs.
Deliberately invoked and Played for Laughs: In Atlanta Nights Bruce Lucent spends a chapter trying to find out if his parents were his bio-parents when it turns out he's black when his parents aren't. Nobody else noticed either.
In The Temporal Void, Troblum has software in his implants that can read other people's facial expressions and body language, since he's not so good at doing it himself. When he notices the people around him behaving oddly in one scene, instead of activating this software (which would be completely inconspicuous and take a split-second), he rationalizes that it's probably nothing and blunders into a trap. Once the Big Bad springs her trap, then he activates the software and sees that, yep, everyone else is scared out of their minds because the Big Bad has been holding them at gunpoint and lying in wait for him.
Although she's by no means presented as being particularly intelligent any other time, Anne Steele (Lucy's sister) picks this up big time in Jane Austen's Sense and Sensibility. She is well aware that Lucy's engagement to Edward Ferrars is a closely guarded secret. But as they're staying in the home of Edward's sister, she decides that she should go ahead and tell her hostess the truth because surely that will work out...
In Pseudonymous Bosch's Secret Series, Cass loudly remarks that a gourmet chef's cooking might be improved with the MacGuffin of the book. The only people who know about its existence outside of her friends are part of the Midnight Sun, and Cass knows firsthand that they're willing to abduct and murder people who expose their plans. When Senor Hugo asks Cass if she knows exactly what the Tuning Forkis, she tells him, which results in Hugo (an agent of the Midnight Sun) abducting her mother. Although this isn't the only time Cass slips up; in the first book, she decides to investigate the Midnight Sun's spa alone by posing as a celebrity, not stopping to think that they might have caller ID, and falls into a trap. Later, she and another character eat pieces of chocolate laid out in the villain's hideout (especially glaring since they know that enchanted chocolate is Hugo's M.O.), getting them captured. Then Cass eats ANOTHER piece at the end of the book, which puts her into a coma.
The Race does this a lot in Harry Turtledove's World War series. To be fair, though, human behavior is as alien to them as they are to us. However, they will mindlessly believe anything their superiors tell them, even when there is evidence to the contrary. Case in point, the Race medics have come up with a Truth Serum that works on humans and have claimed that it is 100% effective. It is then tested by a Race commander on a human, who is obviously lying, and the serum fails. While the commander is still suspicious of the man, he concludes that he must be telling the truth based solely on what he has been told by his superiors.
Actually justified (at least in one case) in David Eddings' Elenium. The Dark God Azash has no grasp of subtlety, and this quality creeps into those who serve him. Thus, formerly renowned Manipulative Bastards are found concocting infantile plots which are undone with incredible ease. They seem to get over this, however, and serve a greater threat toward the end of the series.
Semi-example in the Dungeons and Dragons fourth edition novel The Mark of Nerath. A death knight gets his hands on a vial of mysterious liquid which compels him to do things that are... not in his best interest.
As Noah Antwiler points out, a great deal of agony and stress in Twilight could have been avoided if Edward just let Bella be on top when they have sex, avoiding the problem of him being too strong and breaking her. Given his vampiric super-hearing, he should have known something unusual was going on inside Bella's body before she revealed to him that she was pregnant.
In spite of Carlisle's doctoral degrees, when Bella was starving to death, it took an unschooled haughty teenage boy to tell him that a half-vampire fetus needs blood.
As well as the one Rosalie took on when she took over responsibility for the newborn baby and immediately let her guard down, despite knowing full well that there was a vampire-slaying werewolf in the house.
Sisterhood Series by Fern Michaels: In the book Lethal Justice, Charles Martin uses powerful connections to force reporters Maggie Spritzer and Ted Robinson to relocate to New York and stay there, where they can be constantly watched. That's fine. What's not fine is how the next book Free Fall has Maggie and Ted move back to Washington, D.C., without Charles and the men with presidential gold shields being aware of this development at all! Maybe Charles and those men were not as smart as they believed they were!
A lot of trouble in Magyk could have been avoided if Queen Cerys hadn't taken the Custodians as her bodyguards.
Without Silas Heap opening the Sealed room in the Palace and unwittingly releasing Queen Etheldredda in the process in Physik, the whole plot wouldn't have occurred.
World War Z: A good deal of the entire world (except Israel) seems to have become complete idiots for large portions of the book, by burying their heads in the sand and trying to ignore/suppress the zombie problem until it was too late. Most notably the Battle of Yonkers. The galling part is that many of the same people responsible for that had a Smart Ball before that and later on. The US military suddenly became absolutely incompetent for that one sequence to happen.
The characters in Redshirts are well aware of the Idiot Ball, as the Narrative passes it from character to character for dramatic effect. Lampshaded in that most of the characters realize their actions are completely stupid, if not at the time, then after, yet are unable to help themselves.
Katniss doesn't realize Peeta genuinely had a crush on her and Peeta assumed Katniss's reciprocation was real. Katniss is simply terrible at reading people and when Peeta declares his love for her on national television, most people would have used it to their advantage to get more sponsors.
Plutarch Heavensbee goes out of his way to show Katniss his watch which is a clue for surviving the arena and it doesn't occur to Katniss that the Mockingjay hologram in his watch hints that he is an ally. In fact, even after Katniss does realize it was a clue later on, it never once dawns on her that was the point - even after she hears Plutarch with Haymitch and Finnick on the airship that saves her. It takes Haymitch restraining her, then sitting her down to explain for her to get it.
In the Horus Heresy books of Warhammer 40k The Emperor of Mankind plays the Idiot Ball big time causing his own near death and the downfall of his empire through mistake, misjudgment and a total failure to comprehend basic human behavior much less the emotional needs of his 'sons'.
Warhammer 40K is an idiot setting. Pretty much every major event is driven by people, often on both sides, making terrible, short-sighted decisions that blow up in their faces.
At the end of City Of Glass, Clary decides to wish Jace back to life. Which is great except she seems to have forgotten about all the other shadowhunters that died. Real considerate there, Clary. Though considering what happens it causes Jonathan to come back to life and create an anti-nephilium group, this might not be a bad thing.
She usually grabs the Idiot Ball and runs with it whenever Jace is involved, like when possessed!Jace is able to trick her into mind-control just by batting his eyelashes at her and playing on her hormones.
Jace grabs it with a bloody deathgrip when he refuses to tell anyone about the possible prophetic dreams he's having about murdering Clary until it's almost too late, nearly starves himself to death, doesn't sleep... oh, and ends up getting himself possessed by Sebastian. What an IDIOT.
Simona Ahrnstedt gives us a really painful example of this in her debut novel, ÷verenskommelser. It's the story about young Beatrice, who's bullied and pressured by her abusive and tyrannical uncle into an engagement with a man. A man who's not only like forty years older than her, but he also treats women like they're dirt under his shoes. So what does she do? Of course, she tells nobody the truth about why she agreed to marry this man (he would get her beautiful but weak cousin instead, if she didn't sacrifice herself). To be fair, she really is in a crappy situation, but still, yikes... And unfortunately, her love interest Seth is no better. Since he thinks that she willingly rejected him to marry an old disgusting aristocrat, stupid pride keeps him from admitting that he loves her. Several misunderstandings between them (sigh...) leads to much misery for them both (including that Beatrice gets brutally raped and battered on her wedding night).
Not to mention that while Beatrice is a very intelligent young woman, when it comes to academical and intellectual pursuits, she can still make some very dumb decisions.
Beka's journaling in Provost's Dog is explicitly a memory aid, to the point where her watch commander orders her to start doing it again after her reports get sloppy. However, there are a few points where Beka struggles to remember some key detail crucial to the case. Does she go back over her old journal entries where she has already written about that precise thing? No, she just tries to remember it on her own. Okay.
The comedic fantasy Zeus Is Dead: A Monstrously Inconvenient Adventure, in which the Greek gods (including the nine Muses) return to the modern world, has an actual Idiot Ball. It's usually kept safely in conceptual form in the Muses's Hall of Creative Abstract Concepts on Olympus. Thalia has the other muses coalesce it into physical form for her as part of a plan to free Leif and Tracy from Dionysus's casino. Then she loses track of it. Problems occur.