Third-Act Stupidity

This is when for most of a work, the hero or the villain has been reasonably smart in his/her/its actions. But if that character acts just as smart in the climax, it would be over in a minute. Time for this Sub Trope of Idiot Ball.

For example, a cop protagonist has done just about everything with his/her partner, until it comes time to confront the murderer, and then the cop does it all alone.

Or on the villain's side, this bad guy has been one step ahead of the police the whole time, and whenever a cop gets close, that cop dies. But now it's the climax, so the villain does stupid things to drag this out (even if the villain does come out on top).

A Villainous Breakdown, Honor Before Reason, and Revenge Before Reason might be confused for this trope, but are not, as those are logical progressions of events in the story.

Compare Lowered Monster Difficulty, Why Don't You Just Shoot Him?, Third-Act Misunderstanding, You Can't Thwart Stage One.

Anime and Manga

 * Death Note is an odd example: both the protagonist and antagonist are geniuses playing Xanatos Speed Chess three layers deep. The climax depends on neither of them having a plan D. It might seem unfair, but really, with the amount of prep time and Applied Phlebotinum they had access to, they really should have.
 * Justified in Light's case -- Hubris is the whole reason he eventually falls. Well, that and getting his Writer On Board privileges pulled.
 * Light had also been reduced to great insanity by this point, and ultimately it was Mikami's failure that caused his failure. Had Mikami followed Light's instructions as he had done previously, Light's speech in the end would have been a gigantic Crowning Moment of Awesome
 * Or, y'know, if Mikami had just had the rather obvious foresight (as Light did several times) to keep torn pages / pieces of the Death Note around for emergencies. In a continuum where even the slightest mistake will be punished to the fullest extent, this is the very LEAST of contingencies that would be accounted for.

Film - Animated

 * In Hercules, Hades has managed to get Hercules to agree to give up his super-strength in exchange for Megara's survival and safety. So, what does Hades then do? While he's taking over Mount Olympus, he sends a giant Cyclops to attack Hercules, who will no doubt be with Megara, thus making it highly likely that she will be harmed in the crossfire. And since the deal is immediately nullified if she's harmed, he has effectively insured that his plan will fail.
 * Even better, the reason Hades did all this was because of a prophecy that was stated verbatum, "Should Hercules fight, you will fail." His super-strength was not stated as a requirement. So after sidelining Hercules, he then sends a giant Cyclops to basically ensure that Hercules does in fact fight. And naturally, Hades's plan fails.

Film - Live Action

 * Quite a few thrillers end this way.
 * The original ending of A Perfect Murder (the remake of Dial M for Murder) was an aversion of this. But it was decided that the audience couldn't morally identify with her, and then we had the wife doing this trope.
 * Once Upon a Time In Mexico: The Femme Fatale spots her ex-boyfriend wounded in the street from a distance; the sensible thing to do would be to snipe him, as she was perfectly capable. But instead she goes down to talk to him, so that they could have a dramatic final exchange and.
 * None of the characters in Hellboy II make intelligent decisions in the third act, which lead to  (although critics felt it didn't hurt the overall film).
 * The movie Underworld when the Big Bad has rendered the hero completely helpless and has him on the floor at his feet... but then walks away to deal with another problem. The hero of course recovers and saves the day.
 * Of course, the Hero is the spitting image of his dead daughter - a daughter he'd already been mortified by having to kill before. It was possible he wasn't able to bring himself to deliver the killing strike this time.

Literature

 * Double-subverted in Artemis Fowl: Artemis specifically refers to the "third stage of operations" as the time not to get careless, and then

Live Action TV

 * Harmon Rab and Sarah Mackenzie of JAG are often guilty of this. When it came time for the episode climax, they would often confront the bad guy alone. They are partners, but they seem to forget that when it comes to one of the very reasons law enforcement has a partner system.
 * Especially since they are lawyers and not NCIS agents. Mac was a Marine, and a badass one at that, but still.
 * Charmed Zankou, S7's Big Bad is smart enough to come up with some cold-blooded torture that weakens the sisters' confidence enough so he can steal the Book of Shadows. Then in the finale his IQ seems to plummet and the sisters manage to goad him into doing something stupid so they can attack him. It was a shame because he was one of the few worthy opponents they had up to that point.
 * Part of the fun of Columbo was zig zagging this. He's seem like a fool, and even seem to fall into this, but it's all an act.
 * The Big Bad in the Doctor Who special "The End Of Time" tells the Master in the most insulting terms that the moment his plan (which the Master is an essential part of) succeeds, the Master will be killed. While the Master is standing next to the machine that forms another essential part of the plan. He also passes up numerous opportunities to shoot the Doctor despite having previously shown a willingness to kill people for disagreeing with him; and the Doctor isn't part of his plan.
 * In the original series story "The Brain of Morbius", the Doctor defeats and captures Morbius by the middle of the last episode, deciding to remove his brain and return it to the Time Lords. It's fairly logical for him to threaten Solon, the Mad Scientist responsible for giving Morbius his new body, into doing the job for him. It's less logical for him to leave Solon alone to do the job and go and check on Sarah in a room with a lock on the door. To the surprise of no-one but him, Solon locks them in and revives Morbius.

Visual Novels

 * Yoshiyuki in Da Capo II develops this during the Koko route. While he's never super perceptive about love, in this route he basically goes out of his way to be as incompetent as possible about the matter until the point the stupidity starts to edge into jerkassery. The most likely reason is that there's no real dramatic story set up for Koko; she loves Yoshiyuki, pretty much always has and has nothing seriously wrong with her life.

Western Animation

 * In Avatar: The Last Airbender's finale Zuko tells Katara that he can beat Azula alone, which turns out to be true and is about to one-hit Azula the same way he did a few episodes back with his father (and at that time he was unprepared). Then he makes the mistake of taunting her. Katara makes the mistake of standing too close to the action. Result? Azula fires at Katara and Zuko is taken out of commission shielding her from the lightning, leaving Katara to finish what he started.