Just One Little Mistake

The detective has finally cracked the case, figuring out who committed the crime and how it was done. He gathers everyone together and delivers The Summation, describing in meticulous detail how the crime was pulled off, before finally fingering the perp by name.

The accused looks down in bewilderment, and it's clear to the audience that he's the murderer. Just when you might expect him to break down, confess and launch into a Motive Rant, he looks up confidently, laughs, and calmly comes back with...

"Interesting theory, detective, but where's your evidence?"

The implication is, of course, that while the detective's line of reasoning may be rock solid, and he may have successfully deduced how the Locked Room Murder was committed, he still doesn't have any strong evidence that proves the accused was involved in the crime.

However, the detective can in fact prove it, and has an ace-in-the-hole piece of evidence in his possession that he was just waiting to trot out at the right moment. The detective will reveal that the perp made Just One Little Mistake, and then, much to the accused's chagrin, produce a decisive piece of evidence that completely buries him. Cue the Big No.

This might happen in a Clueless Mystery. Also see Bluffing the Murderer, Pull the Thread.

Anime

 * In Death Note, both L and Near deduce very quickly that Light must be Kira. The rest of the series they spend trying to get Light to make Just One Little Mistake.
 * Detective Conan. Literally almost every episode ends with this, immediately preceding the Motive Rant.

Film

 * Subverted in Hot Fuzz, when Nick Angel accuses Simon Skinner.

Literature

 * The talk between Harry and Voldemort in the end of Harry Potter and the Deathly Hallows. "You made just one little mistake! You didn't know that
 * Delivered oh-so-smugly at the end of Empire From the Ashes to "Mister X", who otherwise might have escaped detection completely thanks to elaborate contingency plans and preparations. The mistake?
 * Although, to give them credit, Collin himself pointed out that it only told them something didn't quite add up. That only got them looking harder. If we were to look at everything, we would probably find other places where "Mister X" slipped up.

Live Action TV

 * The Columbo series did it pretty much every episode, and Columbo's trademark reveal was when he would touch his forehead absently and say, "There's just one more thing."
 * Monk. Very frequently. Some examples:
 * In "Mr. Monk and the Candidate," Monk's clue that the campaign manager is responsible for hiring the assassin who has killed two people is that he knew where the shooter was firing from and even pointed at said apartment though balloons were blocking his view and the gunshot created an echo through the skyscrapers in the area.
 * In "Mr. Monk Meets Dale the Whale", the crucial mistake is that the killer, who was supposed to be a fat and heavy man, did not break the chair he stood on to disconnect the smoke alarm despite his weight..
 * In "Mr. Monk Is on the Air," the jockey's mistake in hiring a dog to rig his wife's death was that the dog took one of his shoes.
 * In "Mr. Monk's 100th Case," the second killer, trying to frame a serial killer for the killing of his girlfriend, made the mistake of strangling the victim from behind, not from in front like the other victims.
 * In "Mr. Monk Gets Hypnotized", the mistake Monk calls out on in murdering her husband is that the piece of gum he found stuck on her shoe is a piece of Randy's chewing gum that Stottlemeyer had spit out when they were questioning Sally's husband at his house, proving that she had actually been back to her house and had not been chained up to some floorboards in a rural cabin for three days.
 * It was a mistake of the biologist's assistant in "Mr. Monk on Wheels" to wear tire sandals when she killed her accomplice because of the footprints.
 * Law and Order: Criminal Intent interestingly inverted this in one episode, in which Goren let it slip to a suspect's wife that their recovered body was missing a tooth cap. The perp, who prided himself on clueless murders, proceeded to tear his garage apart looking for the tooth cap, and the detectives strolled in in the morning to see him cackling madly about dental equipment. Of course, so maybe this is more of a subversion.
 * In any case, the trope was played with.

Video Games

 * Phoenix Wright: Ace Attorney features this regularly. Sometimes subverted in that a number of later cases penalize you in this situation if you claim that you have evidence. The correct solution is to say that you have no evidence and let Phoenix angst his way through the next few seconds until something turns in his favor, often alongside being admonished for giving up... right after a sequence in which giving up is the only valid option.