The perp would have gotten away with it if not for one minor slip-up.
Let's say 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!".
- Bleach's first Big Bad, Sosuke Aizen, might very well have won had he not, as a demonstration of his new "evolved" power, killed the supposedly indestructible "janitor" of the Precipice World to show that the laws of reality were now beneath him. Isshin and Ichigo Kuroski take advantage of this to give Ichigo months' worth of training in the space of a few hours, as the Precipice World is subject to Time Dilation; ordinarily, the janitor would kill anything in there that long, but Aizen had already killed it. When Ichigo emerges, he is far stronger than Aizen. Given Aizen's nature as The Chessmaster he might very well have intended this, as he'd been pitting his new powers against successively stronger opponents to evolve them further, but even so it didn't end well for him.
- Case Closed. Literally almost every episode ends with this, immediately preceding the Motive Rant.
- 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. Which he eventually does.
- Subverted in The Last Seduction, when the imprisoned Mike realizes that Bridget made one mistake in her master plan involving a name tag on a mailbox. The film ends with Bridget slipping the tag out of the mailbox and driving off in a very pricey car.
- Subverted in Hot Fuzz, when Nick Angel accuses Simon Skinner. The trope is set up perfectly and Angel even triumphantly announces his evidence of Skinner's Just One Little Mistake - a cut on his leg sustained in a chase. Except the cut isn't there, and Skinner smugly hands over evidence that he was in his grocery store all day.
- In 36 Hours (1965) WWII German agents use a Faked Rip Van Winkle deception on a captured American officer in an attempt to learn the details of the upcoming D-Day operation, including changing his body while he was unconscious to make him look and feel six years older. He notices that he still has a minor injury that ought to have healed if so much time has really passed.
- The League of Gentlemen: The "little mistake" that blows an otherwise perfect bank robbery wide open is that one of the crooks brought his own car to the scene of the crime, which then had its license plate written down by a kid with a hobby of collecting license plate numbers, which he then gave to the police, who investigated all of the people with cars that were near the scene of the crime.
- Mission: Impossible: The moment in which Ethan Hunt starts to suspect that Jim Phelps is still alive and is responsible for the frame job that has put him on the run for the whole film is when he opens the Bible he took from the Prague safe-house and sees the "Placed by the Gideons in the Drake Hotel - Chicago" stamp on it, recalling Phelps' mention of going to that hotel the day before the Prague op.
Jim Phelps: Those damn Gideons.
- Subverted in Match Point. The killer clumsily disposes of the ring he stole from his first victim, only for it to end up in the possession of a homeless man who dies shortly afterwards. The police rule the killer out because of this. One of the detectives wonders if he really did do it, but he can't prove anything.
- The talk between Harry and Voldemort in the end of Harry Potter and the Deathly Hallows. Paraphased it goes something like this. "You made just one little mistake! You didn't know that the ownership of the unbeatable wand transferred from Dumbledore to Draco Malfoy, and from him to me, giving me instant victory in this duel!"
- 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? Making absolutely no mention of orchestrating the heirs' assassination—what should have been a crowning achievement—in the supposed diary of the guy taking the fall.
- Although, to give them credit, Colin 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. Then again, Colin also notes that it was that little mistake that got them to write off the guy taking the fall, and made them look at other people which could have pulled off what they know was pulled off... which was a very short list.
- Encyclopedia Brown was this, aimed at kids.
- Agatha Christie uses this trope in several novels.
- The Dr. Thorndyke mystery novels use this trope; one in particular, Mr. Pottermack's Oversight, is named after it. (Pottermack's one little mistake was overlooking a tiny but crucial detail about his victim's shoes when he faked the footprints that led away from where the murder actually happened to the place where the body was finally found.)
- 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."
- CSI: In the episode "Killer", an ex-convict kills his former accomplice after the latter confesses to be the man who testified against him in order to get a reduced sentence of his own. He has an elaborate plan to get rid of all evidence, but while driving away his car is struck by a young drunk driver, who insists on calling police even thought the ex-convict tries to talk her out of it. He kills her to prevent this, but two murders happening within a few blocks of each other are so rare an occurence that the CSIs decide that it could only be the work of a single killer, and he further outs himself when he wires some money to the second victim's family out of guilt. By the time the police identify him, he's leaving town already, but he surrenders himself when his wife tells him that they are threatening to take the custody of their daughter from her.
Karl Cooper: So, tell me ... where'd I go wrong?Grissom: You killed two people.
- CSI: NY: The only mistake the second killer in 'Criminal Justice' makes is he planted the evidence after Hawkes had sprayed for footprints at the scene, and the distribution of chemicals on the evidence alerts the team to the fact that it was planted later. Otherwise he nearly commits The Perfect Crime. Which makes sense, because he's a DA, and has fifteen years of experience with criminals and the crime lab to know how they work. It's also a case of Murder the Hypotenuse because the planted evidence is a lighter belonging to his wife's lover; he killed that guy and ground down his body attempting to invoke Never Found the Body.
- In an episode of M*A*S*H, circumstantial evidence points to Hawkeye being responsible for a series of petty thefts around the 4077th. He lets it be known that the stolen items have all been collected in one location for fingerprinting, anticipating that the real thief would come for them to conceal the evidence. When the evidence goes missing that night, Hawkeye gathers the staff and informs them that the stolen items had all been covered with a chemical compound that would turn the thief's fingernails blue. Everyone in the tent looks at their hands — except for Ho-Jan who immediately hides his hands under the table. He then holds them up, happily showing his fingernails are normal, but it's too late. Hawkeye had been bluffing about the chemical and was just watching to see who would panic.
- 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. That's because he was the fat man's accomplice, wearing one of his empathy fat suits.
- 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 Sally Larkin 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.
- On "Mr. Monk and the Twelfth Man", the 'little mistake' for the initial killing of a man's wife was leaving her wedding ring in plain sight on a construction area (in which he was burying the body), where a member of a jury could take it and use it for blackmail. The mistake only got bigger when the man decided he was fed up with paying the blackmailer (the juror was a gambling addict, you see) and decided to kill everybody on the jury.
- "Mr. Monk Visits a Farm": After Randy has finished reeling off the explanation Monk fed him while he was sleeping, Belmont gets in his face and claims it's just an unproven story. Then Randy asks the local law enforcement for a pen and an evidence bag and retrieves the key from the pickup. No one ever moved it after the "suicide", so if Belmont's fingerprint is on the key, it proves he handled the pickup last.
- Law & Order: Criminal Intent exploited 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, turns out there was no tooth cap.
- According to Sherlock, the murderer always makes a mistake. In the aptly named first episode, A Study in Pink:
Lestrade: What mistake?Sherlock: PINK!
- Castle is fond of this. They'll get the suspect pinned down (usually in the interrogation room, sometimes not), and Castle will explain what happened. The suspect will smirk, say something like "That sounds like a very good story, something out of one of your books," and then Beckett will pull out the witness statement or DNA evidence or whatnot and the smirk fades.
- In the Midsomer Murders episode "Faithful Unto Death", the killer plants evidence of her crime in her friend's purse right before Detective Barnaby gives The Summation, then wryly asks him if he has any evidence. He tells her that he had searched that same purse shortly before she arrived, then looks in the purse and pulls out the evidence, prompting her to incriminate herself. On the way out, he admits that he was bluffing.
- James Gray, a.k.a. The Muralist, from Hannibal murders his victims by administering a lethal dose of heroin. His final victim is a recovering heroin addict who was able to survive the drug and escape. The man does succumb to his wounds shortly afterwards but is able to get far enough away for his body to be discovered by the FBI who are able to backtrack to where Gray was storing his other victims.
- In The Pretender, Ms. Parker tracks Mr. Lyle to the hotel room he last stayed in and then finds out his present location by hitting redial on the phone (a trick that's rarely invoked anymore). It's such a simple thing, yet so easy to overlook, that Mr. Lyle can't help but smile.
- Monty Python's Flying Circus:
- The Lemming of the BDA note sketch inverts and lampshades this as he tells he had the Big Cheese and his gang in a cupboard until his men could come for them due to those naughty dentists making that "one fatal mistake."
- Parodied in the Agatha Christie Sketch [Railway Timetables], in which the murderer's "little mistake" is trying to lie that he was riding a train as an alibi — much to the man's eventual exasperation, all of the investigators in the room have a near-omniscient knowledge of train facts and everything he makes up is instantly detected as a lie.
- 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.
- The notorious Son of Sam Serial Killer was caught because his car got a parking ticket, which gave the police a vital clue to find him.
- Australia's "Lesbian Vampire Killer", Tracey Wigginton, was only caught merely because she accidentally dropped her credit card.
- James Earl Ray, the murderer of Martin Luther King Jr., intended to flee to apartheid-ruled Rhodesia, which besides treating blacks like Ray thought they should, also had the added attractive of not having an extradition treaty with the US. He made it to Canada, then to Britain, and then to Portugal, one of the few countries in the world with diplomatic relations with Rhodesia and, through its colony in Mozambique, also a long border with it. Unfortunately for Ray, the guy who forged his Canadian documents wrote his surname as "Sneyd" in his birth certificate, and "Sneya" in his passport, and the Portuguese sent him back to Britain. Upon arrival he was recognized as the now infamous Ray, arrested and extradited to stand trial in Tennessee.
- While Ted Bundy had already been named as a suspect in his serial killings due to his similarity with a suspect sketch and having the same car model, he only started having problems when he run a stop sign and was pulled over by a traffic cop. The cop then found a mask, gloves, a crowbar, handcuffs, and other items likely to be used in a home invasion inside his car.
- Just as it happened in American Gangster, Frank Lucas got onto Richie Roberts' radar because of the extravagant clothes he wore to the Ali vs Frazier fight.
- The Watergate scandal came to light after a nightwatchman noticed a piece of tape on a door in the Watergate complex, removed it, came back later and noticed it had been reapplied.