This is when an impersonator has an almost perfect disguise, only to ruin it with a seemingly inconspicuous mistake. Perhaps the impostor blurted out something out-of-character (or a personal catchphrase), forgot to include a Distinguishing Mark (often an Intimate Mark), or accidentally revealed that they're left-handed. This can more easily occur during a round of Bluff the Imposter, and can be a sub-trope of Spot the Impostor except that their target generally has no advance knowledge that there is an impostor in the first place.
Related to Pull the Thread, as this is often what leads to it. Compare Saying Too Much, a more incriminating version. See also Conviction by Counterfactual Clue, when this gets even more unrealistic, and Conviction by Contradiction, which overlooks that there may be a perfectly reasonable explanation for the discrepancy. Often overlaps with Imposter Forgot One Detail.
See also A Glitch in the Matrix or a Revealing Continuity Lapse, for when dreams or illusions are given away by their imperfections, I Never Told You My Name and I Never Said It Was Poison for when the impostor gives themselves away by revealing they know something they shouldn't, Suspiciously Prescient Planning for when excessive planning for something one could not reasonably anticipate gives away a crime, or Confronting Your Impostor, which is what happens when somebody stumbles upon somebody else pretending to be them.
- The Adventures of Huckleberry Finn: Huck goes in disguise as a girl at one point; the woman he's talking to gets suspicious when he attempts to thread a needle by putting the needle onto the thread, whereas any girl would know that it is far easier to do it the other way around, and ultimately is found out when he catches a ball of yarn tossed to him via closing his legs such that it lands on his lap. A girl would instead open her legs to catch it on her skirt. In short, Huck didn't do his research.
- Agatha H. and the Siege of Mechanicsburg: When Klaus claims that Gilgamesh was wasped before he even met Agatha, Gilgamesh claims she never gave him any orders. Boris immediately realizes that she did—she politely asked them all to leave (which might not have counted) and screamed a Big "NO!" (which definitely counted). The fact that Gilgamesh easily ignored both orders means he can't have been wasped.
- In Animorphs, Jake winds up infested by a Yeerk after falling into a Yeerk pool. When "Jake" spots Ax (an Andalite, whose species are at war with the Yeerks), his face momentarily contorts into a look of hate, alerting everyone to the problem; the Yeerk blows it even worse when Ax touches him, at which point he yells "GET YOUR HANDS OFF ME, ANDALITE FILTH!". The other Animorphs also point out that if Jake was really the one "in the driver's seat", he would have gone along with their plan to starve the Yeerk just to make absolutely sure and settle everyone’s minds. But since the Yeerk was fighting them every step of the way, the other Animorphs tied him up, knowing with 100% surety that he was a Controller.
- In the BattleTech novel The Sword and the Dagger, it's revealed that Hanse Davion, the First Prince of the Federated Suns, has been replaced with an impostor because the impostor can't pilot Hanse's personal Battlemaster, since the neurohelmet a mechwarrior wears to give their mech balance has to be carefully calibrated to the mechwarrior's brainwave pattern.
- In The Book of the New Sun, Severian spots a literal thread: a ribbon sticking out from the side of the head of the mysterious soldier he battles at the Sanguinary Fields, which should tip him off that it's really Agilus in disguise. Severian spots the clue, but cannot remember where he has seen such a thing before until two chapters later, after he has almost been killed in the totally unnecessary duel. It's a tip-off to the reader. As a famously Unreliable Narrator, because he's not as clever as he appears, Severian often mentions threads, but fails to pull them. And if Severian can't unravel this clue, how can he be expected to unravel the fact that he's being manipulated by time-traveling aliens?
- Books of the Raksura: The mysterious Raksura species can shift between draconic and human-looking forms at will. While flying cross-country, Moon and Stone shift to their human forms to have a polite but strained conversation with some travelers; afterwards, they realize that being in the middle of the wilderness without any traveling gear was a dead giveaway that they weren't really human.
- In Bulldog Drummond, Drummond is able to recognise the Comte de Guy as Peterson in disguise, though he looks completely different, because he has the same unconscious mannerism when he's feeling impatient.
- Clive Cussler is fond of this in many of his Dirk Pitt Adventures books from Spot the Impostor to figuring out clues behind some event.
- In Iceberg, Dirk Pitt instantly knows something is up with Kristi, as she is supposed to have spent years living in the jungles of South America, but her tan is far too shallow.
- In The Mediterranean Caper, Pitt finds it odd that Bruno Von Till, who boasts of his past as a World War I combat pilot, would have a model of a submarine in his study. This leads to his investigation that Von Till is actually wanted Nazi war criminal Admiral Eric Heibert. The same novel has Pitt managing to spot the clues to tell him cop Darius is The Mole for Von Till.
- Night Probe has former MI-6 agent Shaw (who may or not truly be James Bond) brought to his old headquarters to be recruited into a mission. After agreeing, Shaw asks why they bothered using a building that quite clearly has been deserted for years from the amount of dust around to the lack of a regular staff. The head agent admits it was both to set Shaw at ease and test his observation skills. After a younger agent mocks his age at an airport, Shaw puts him in his place by pointing out how a "businessman" has been following them for the last ten minutes to a flight attendant at a terminal where her airline doesn't have any flights.
- In The Oregon Files novel Skeleton Coast, Sloane Macintryre shocks Juan by being the first person to see through the Oregon's facade of a run-down tramp ship. She's spotted several of the crewmembers wearing clothing or watches far too expensive for a crew barely scraping by. Also, having grown up around boats all her life, she knows there's no way the Oregon can be moving so fast without highly advanced turbines developed after it was built. The fact the ship was moving when the bridge telegram was at "all stop" and it took several minutes for the "smokestacks" to billow out smoke added to it. The clincher is her pulling back a painting on the wall to reveal a hidden camera as she'd seen Juan glancing to it a few times during their talk.
- In Zero Dawn, Pitt has a computer search uncover that over 80 people died on April 18th, 1906, all at almost the same time at different places but each obit almost exactly the same with just a few words changed. They all actually died in an experiment gone wrong (which inadvertently caused the San Francisco Earthquake) and the government hushed it up. The government agent dryly states that in 1906, no one was really considering the possibility some fancy machine would be able to see through the pattern.
- The Isaac Bell series focuses on a detective who's a master of spotting the tiny details (such as figuring out a man faked being knocked out in a fight as his eyes are still very alert rather than showing the effects of a blow).
- In Codex Alera, Magnus figures out that First Spear Valiar Marcus is actually the traitorous Cursor Fidelias by spotting a number of such threads, such as Marcus being able to reliably spot him when he's trying to be stealthy, and seemingly having disappeared off the face of Carna after serving his military term (because Fidelias abandoned that cover identity after serving his time, later reusing it to spy on the First Aleran).
- The Dark Descent of Elizabeth Frankenstein: On her wedding night with Victor, Elizabeth realizes that the Monster couldn't have murdered William or framed Justine, as the handprints on William's neck are far too slender to be his. Having earlier discovered Justine's body in Victor's custody during his trip to England, Elizabeth quickly figures out the real murderer was Victor, who framed Justine in order to claim her body for his experiments.
- Basically the reason the fake meteorite is exposed in Dan Brown's novel Deception Point; all the evidence supported the idea that the meteor in the iceberg was real, but once they discovered a few microscopic fish under the meteorite, introducing the idea that it had been planted in that iceberg on purpose, Rachel, Michael and Corky are able to take another look at the evidence and admit that there are other explanations for the evidence beyond it coming from outer space.
- In Do Androids Dream of Electric Sheep?, Deckard is initially led to believe that he's registered a "false positive" on Rachel Rosen's Voight-Kampff test (ie. he's identified a real human with stunted empathy as an android, something that would have invalidated the entire testing process and forced Earth's police agencies to come up with a whole new test from scratch). The Rosens, apparently looking to protect their own reputation, try to buy his silence on the matter by bribing him with their (very rare) pet owl, but when Rachel continuously refers to the owl as "it", instead of "she", Deckard gives her one final Voight-Kampff test and confirms that he's been had: Rachel is an android, and the Rosens were trying to set him up to pressure him into dropping his investigation.
- In Double Star by Robert A. Heinlein, an actor is hired to double for a prominent politician who has been kidnapped, and spends weeks studying the politician's files on everyone around him to make the masquerade work. He notices that the file on the emperor is nearly empty, and assumes that they don't have much contact...until he meets the emperor, and gets found out, because why would you need files to remember how you deal with your longtime friend? He's caught when he dutifully plays with the emperor's toy trains instead of teasing him about them.
"It was not until later that I realized that the file had not been defective, in view of the theory on which it was based, i.e., it was intended to let a famous man remember details about the less famous. But that was precisely what the Emperor was not—less famous, I mean. Of course Bonforte needed no notes..."
- The Dresden Files: In Battle Ground Ethniu shows Harry a memory pulled from her mortal soldiers showing them entering the Carpenter home and massacring everyone inside, including Maggie. Harry quickly figures out it's a setup since the memory has no sign of Mouse. This thread was deliberately left by Molly when she designed the illusions the soldiers "killed" so Harry would see through Ethniu's manipulation.
- In Dale Brown's Edge of Battle, Jason tries to disguise himself as a Mexican to infiltrate a meeting by the villains, which works until one mook notices that he has a gringo's blue eyes.
- Discussed in The Emperor's Soul by Wan ShaiLu, a Master Forger whose work requires her to be a polymath with an exacting eye for detail. When she learns that the Emperor is brain-dead and a government faction is trying to hide that fact by keeping him "in seclusion", her first instruction is that someone regularly fill the Imperial chamber pot, because servants would notice if he stopped using the toilet.
- In the Georgette Heyer novel False Colours, Kit is caught in a Twin Switch when he opens a snuffbox the wrong way.
- Garrett, P.I.: In Faded Steel Heat, Garrett is impersonated by a shape shifter, who visits Morley Dotes' restaurant to find out how much Garrett's friends know. The phony is exposed almost immediately, as Morley's waiters offer "Garrett" a platter of roasted green peppers, and the fake tucks into it at once. The offered platter had been a prank, as steak-loving Garrett has spent several books denigrating Dotes's vegetarian menu, and green peppers in particular. Note that they did know shape shifters were around, so it wasn't that huge leap from 'Garrett acting unusual' to suspecting it was a 'shifter. Flushing them out was actually one of the reasons for feeding him that meal.
- In Ghost, when looking for a nuclear bomb in the press parking lot at Notre Dame, where the Pope was scheduled to speak, Mike identifies the enemy agent when he claims to be from Alabama, but is eating his meal in the Continental style (not switching the fork to the right hand to cut with a knife in the left) instead of the American one.
- Ghost Roads: Bethany gets Rose a wrap for her dress, so she'll have a "coat" that won't stand out at the dance. She says she was able to match the color from the yearbook picture. Thing is, though, Rose died before she made it to prom, so there aren't any pictures of her in her dress. This is an early clue that Bethany is working with Bobby Cross.
- Harry Potter
- Harry and Ron make some errors when they turn into Crabbe and Goyle in Chamber of Secrets, although Malfoy apparently doesn't think anything of it other than that his friends are acting a bit weird today. For example, they forget that they're supposed to laugh at Malfoy's jokes and Ron corrects Malfoy when he gets Percy's name wrong.
- In Goblet of Fire, Dumbledore realized that something was up with "Moody" when he removed Harry from Dumbledore's sight—the safest place he could be—in the midst of all the chaos surrounding Cedric's death.
- Harry Potter and the Deathly Hallows: At Fleur and Bill's wedding, Harry uses the Polyjuice Potion to disguise himself. It doesn't fool Luna Lovegood, who recognizes him from his expression.
- A villainous example: In Hallows, the Death Eaters realize which one of the several Harrys are the real one when he uses his "signature" spell, Expelliarmus.
- Also in Deathly Hallows, Harry, Ron and Hermione go to visit Xenophilius Lovegood in order to find out more about the Hallows. To their surprise, Lovegood is rather reluctant to talk to them, even though he had been using his magazine to support Harry. All through their meeting, Lovegood acts very strange, and it isn't until Harry goes to Luna's bedroom and notices a thick layer of dust on the furniture that he realizes Lovegood had been lying to them about Luna being down by the river in order to get them to stay put long enough for him to call the Death Eaters so they could be captured.
- Honor Harrington:
- In Echoes of Honor, a Haven admiral notices that the prison planet Hades has been taken over because the warden has been playing a game of chess with him (move by move, by courier ship), and the warden's move doesn't arrive when it should.
- It's worth noting that the rest of his command staff thinks he's insane for sending a whole battle fleet to investigate on the basis of a missing chess move. He is, however, right.
- Used deliberately when a Solarian supposedly assisting Haven's State Sec digitally alters the uniforms and bridge of the State Sec crew to look Manticoran when they communicate with an Andermani warship. The Andermani commander realizes immediately that someone is trying to con him because the Solarian secretly inserted a CGI "extra" in the background of the bridge, one that looks exactly like Honor Harrington. Not only did the commander know her personally, having met in the past, at the time she was thought dead after being executed by the People's Republic of Haven.
- In Echoes of Honor, a Haven admiral notices that the prison planet Hades has been taken over because the warden has been playing a game of chess with him (move by move, by courier ship), and the warden's move doesn't arrive when it should.
- The Hunger Games: The official, "live-action" shots of District 13 are revealed to be Stock Footage by a mockingjay which flies past the screen at the exact same spot despite claims that it is filmed repeatedly every year.
- House of Robots: Robots Go Wild!: Sammy notices that Eddie Ignalls keeps getting the name of his brother, Freddie, wrong. Whenever he points it out, Eddie covers it up by saying it's his middle name or a nickname. In truth, there is no Freddy Ignalls. It's just a ruse to get SS-10K into the school and close to E so he can sabotage him.
- The Impossible Us:
- The Berenstain Society is united by its members' recollection of The Berenstain Bears in place of The Berenstein Bears, in whose timeline they actually live.
- Nick and Bee notably fail to do this during their initial correspondence. Once Nick finally gets suspicious, he determines he's overlooked fifty-four discrepancies between Bee's i-mails and reality.
- In Into the Drowning Deep, sirens can perfectly mimic any sound they hear, but they can only repeat it exactly and have no sense of the context of the sounds most of the time. As such, they can be found out when they do things like mimic the sound of a boat motor a mile under the surface or repeat the same whale song regardless of circumstance.
- Maximum Ride's clone might have been a more successful replacement if she had known that the blind guy was the Team Chef, but given that the flock's youngest member is a freakin' mind reader the chances are slim.
- Night World: In Huntress (1997), one of the reasons Morgead figures out that Jez is a Daybreaker before she tells him herself is because of her use of the phrase "Oh my goddess", as opposed to "oh my god"; he notes that this is a phrase more commonly used by witches (the founders of Circle Daybreak) and thus Jez must've been spending a lot of time around them.
- In the Philip K. Dick short story Precious Artefact, the reconstruction engineer first suspects on returning from Mars that the Earth streets he's seeing aren't real when his coin falls through a dispenser. This is later confirmed for him when his houseplant, in Earth's supposedly more humid environment, dies of dehydration.
- Rivers of London: In The Hanging Tree, the sight of an expensive mechanical watch on Martin Chorley's wrist — one without microchips that can be destroyed by magic — tips Peter off that Chorley is almost certainly the Faceless Man.
- Discussed in the Agatha Christie novel Sad Cypress. Hercule Poirot explains that while most criminals are smart enough to create elaborate alibis about the crime, they'll often tell rushed lies to cover up small details that they hadn't thought of beforehand. Poirot goes on to say that telling a stupid, easily-disproven lie is often enough to at least get people questioning the suspect's honesty, which in turn leads them to panic and further incriminate themselves. In this case, the thread is a small wound on the murderer's wrist—they claim it's from a rose tree, but the roses on that particular plant don't have thorns. It's actually the mark of a hypodermic needle which the killer used to inject an emetic and vomit up the poisoned meal they'd eaten with the victim.
- In Santiago: A Myth of the Far Future, Cain is caught in a vision of being on his home planet by Altair of Altair. When he's asked by Altair, in the guise of a girl he had a crush on, to drop the "stick" (his gun) and help her across a brook, he shoots her and tells the corpse that his home planet doesn't have any brooks.
- In Shaman Blues, Witkacy spots that something doesn't quite match in Tadeusz Dzwon's story by the man's brief terror at the very mention of his sister, putting the detective on fast track to figuring out the murderer.
- Star Wars Legends: Galaxy of Fear has a spot of this in Clones. Our heroes have been separated and Tash is on the run from a number of her own clones, commanded by a clone of Darth Vader. When she's menaced by a clone of Hoole, she's saved by Zak, who then wants her to punch in a code to summon their ship...but then she remembers that both of them are supposed to know the code, so why hasn't he done it? Yep, he's a clone too, and goes on the attack as soon as she figures it out.
- Agatha Christie's Tommy and Tuppence book N or M?: Tuppence does this almost literally. Her fake identity, "Mrs. Blenkensop," is not good at knitting, but Tuppence is. At one point, she gets distracted and Mrs. Blenkensop suddenly starts knitting perfectly. She tells the others who witness this that she was pretending to be a poor knitter so one of the other residents at the hotel would have an excuse to help her.
- Tortall Universe:
- Aly does her best to avoid mistakes of this kind in the Trickster's Duet and pretend to be a simple country maid while posing as a slave in the Copper Isles instead of the highly educated daughter of Sir Alanna, King's Champion of Tortall. This is made difficult when Sarai and Dove don't want to hear about anything but Alanna, and Dove notes every slip.
- Beka Cooper:
- An illegal slave sale in Terrier is found out by the spotting of several such threads in the announcement they sent out. The threads were these: They used pure red wax for seals (the legitimate version includes ebony shavings), used a cotton ribbon (legitimate announcements use silk), misnamed the king, and wrote the letter "s" normally (scribes at the Ministry of Slave Sales put an extra curl on the letter "s").
- Bloodhound: Beka notices a man trying to exchange counterfeit silver coins for copper ones because it's too early in the evening for most Lower City residents to have silver (gambling starts later at night, and gambling is the only real way for most to get silver). When she investigates further, she notices that the profile on the coins is pointed in the wrong direction. The man is quickly caught.
- In The Unexplored Summon://Blood-Sign, Kyousuke first suspects that Aoi is actually the White Queen when she doesn't forget about him when he's out of her sight (in the setting, normal people can't remember anything about summoners outside their field of vision). This wasn't certain evidence on its own, since Aoi is an Artificial Human to which this might not apply. The key evidence is "Aoi" using a specific technique for squirting water with one's hands, which Kyousuke originally taught to the White Queen. However, it's implied that these slip-ups may have actually been deliberate, given that the end of that novel reveals that the White Queen wanted Kyousuke to defeat her.
- Vorkosigan Saga: Invoked at one point in Cetaganda, when the Barrayarian Embassy's "protocol officer" Colonel Vorreedi has a talk with one of the young officers sent to officially represent his homeworld at a State Funeral. With a few leading questions on both sides he swiftly sees the truth behind Miles, Lord Vorkosigan's cover as the Prime Minister's half-crippled son being given a sinecure to keep him out of everyone's hair...aside from the detail that he was not at the moment on a top secret covert ops mission at the behest of ImpSec director Simon Illyan himself.
- In The Warrior's Apprentice, one character video-calls his friend and ally. The call keeps having static in between responses. Miles notices that one object on the desk moves between responses, and correctly deduces that the call stitched together from multiple recordings, and the static is to cover up the cuts. The guy they're "talking" to was captured long ago.
- Warhammer 40,000: Eisenhorn:
Eisenhorn: You could talk like Nayl and use Glossia thanks to Jekud, but Jekud didn't know what Nayl knew. We fought Sadia on Lethe Eleven, not Eechan.
- Twice in The Westing Game, both involving Sandy McSouthers:
"Hi Sandy. I won!"
- Sandy limping from an injured leg has people thinking Turtle, known for kicking shins, had kicked him. She never did...but she had kicked Barney Northrup, pointing to them being the same person.
- Theo had Doug spy on the chessboard he was playing a game with to see who his mystery opponent was, and is surprised to find it was Sandy, given how he said he didn't play chess...but Judge Ford recognized the last move Sandy played-the Queen's Sacrifice-because she's been on the receiving end of it herself, and realizes that Sandy is also Sam Westing, the allegedly deceased millionaire whose game the heirs are playing.
- Both these threads are important, as the real game is figuring out the connection between these three identities and finding the fourth one. Turtle is the only one to figure out that they have Theme Naming (specifically, cardinal directions), and the fourth identity is the man currently running Westing's company, Julian Eastman.
- In The Witchlands, Cam realizes that Vivia might not be the person behind the assassination attempt on Merik when she learns that the Empress of Marstok, whose well-being doesn't interest Vivia in the slightest, has been attacked in the same way.
- In a Russian fantasy novel Wolfhound the titular hero exposes a disguised assassin when he notices that although the man wears a distinct native costume, the patterns on his shirt and boots do not match and indicate him as a member of two different clans at the same time.
- In Keith Laumer's Worlds of the Imperium, the protagonist is recruited as a spy because his counterpart in an alternate universe was a ruthless dictator, and the plan was to replace his double to allow for a successful revolution. The plan works and he's successful in convincing everyone he's the dictator...up until the moment, flush with how successful the masquerade has been, he stands to greet one of the dictator's confidants and is immediately exposed. No one outside the dictator's immediate circle knew he's lost the use of his legs.
- Mistborn: The Kandra can easily shapeshift into anyone whose body they've eaten, except for their hair, which they need to transplant strand by strand. In Wax and Wayne, Wayne muses that he should keep an eye out for someone without any hair on their..,arms.
- In a later book, when the crew is searching for a treasure in a temple, the last chamber contains an empty pedestal next to some broken glass. Wax realizes that this was only meant to look like a robbery when he measures the broken glass, and finds that if the glass case was reassembled, it would be too big to fit on the pedestal.
- In Outpost, Striker and Hop approach Councilman Golm during his regular lunchtime park outing. After realizing the Councilman is likely the bait, Striker looks around, initially seeing nothing out of the ordinary. Then he notices four things: there are no kids in the park, even though it's a popular place for school field trips; the two old ladies walking past them were carrying bags that looks far too heavy, and they themselves looked more muscular than their wrinkles would suggest; a young couple was sitting on the next bench over, even though lovers generally preferred to find isolated spots; and there were three blue-collar workers in a central city park, wearing coveralls, in the middle of a workday. Striker and Hop barely manage to get away, and only because of this and because Fiddler was acting as a Friendly Sniper.
- The Alice Network: Rene realizes Eve isn't from the French region she claims based on her accent. She allays suspicions by lying that she traveled from there for work, and didn't want to say anything since that's very near the German border, thus many French people would suspect that she's a collaborator.
- Cat Chaser: Moran figures out Nolen Tyner is hiding something when he notes that Tyner's listed address is an office building. He also catches on to Rafi's scam because he was left-handed, when the man he had shot was right-handed.
- Discworld: Multiple cases in the Watch books alone.
- When dealing with Inigo Skinner in The Fifth Elephant, Vimes concludes that his walk is so visibly harmless that it's suspicious and lobs an orange at him; it bounces off Skinner as he acts shocked that Vimes threw the orange at him. This makes Vimes even more suspicious of him, though, as most people would either catch or dodge (confirmed by throwing an orange at another member of the Watch, who reacts appropriately), but Skinner saw in an instant that it wasn't a threat. Then he sends Skinner's description back and gets confirmation that he's an Assassin.
- Also subverted in The Fifth Elephant; the replica Scone of Stone is made of plaster and anyone who takes a close enough look should spot it. Albrecht, in accordance with tradition, signs off on the fake Scone as genuine anyway. It's already been replaced multiple times, and Albrecht's family knows it; one more isn't going to be a problem.
- Anyone with a reasonable ability to read people spots that Vimes is used to more serious command than being "Sergeant-at-Arms John Keel" in Night Watch Discworld.
- In Snuff, Vimes spots that someone is masquerading as a sailor because an actual sailor would have sea-legs and be more functional on a rough sea.
- In The Truth, an attack on the Patrician leaves a lot of people looking for the Patrician's dog as a crucial piece of evidence from the scene. William puts out a search request in his newspaper, and is approached by two clerics offering to help him in his attempt. William sees through them when they know what colour dog they're looking for, despite the paper only being able to print black-and-white. This means they've already seen it, probably while committing the crime.
- In Ascendance of a Bookworm, Lutz realizes that given Myne's background, she shouldn't know how to create paper, hair ornaments or shampoo and calls her out on it. She doesn't deny it, but he eventually decides it doesn't really matter who she is since the original Myne isn't coming back and he never properly knew her anyway.
- The Witch of Knightcharm: The protagonist Emily can't find Lauren at first in the packed Club Bacchus-Dionysus. But then she notes that, even though the place is full to bursting with a huge line outside, there's a booth that's open and that nobody seems to want to sit in. Emily realizes that Lauren is there, using magic to make herself invisible and to redirect everyone else away from it, and she's able to approach it and find Lauren just before time runs out.
- In Classical Mythology, this is what leads to the nymph Echo's tragic fate. She's a known Motor Mouth who can't resist getting the "last word" whenever she talks to someone else. One day, she encounters Zeus having one of his many trysts in the woods—and then runs into Hera, who's angrily looking for her cheating husband. Echo covers for Zeus by luring Hera into a lengthy conversation, giving him time to send his lover away. Hera leaves, and Zeus thanks Echo for her kindness by giving her a valuable ring. When Hera comes back to ask Echo one final time if she's seen Zeus, she claims innocence...but Hera notices the ring she's now wearing and realizes what happened. Furious, the goddess curses Echo to only be able to repeat the last word that other people say for all eternity (the echoes we hear in canyons and cliffs are the lamentations of Echo herself).
- In episode 11 of Sequinox, it seems like the girls have gotten back to their world. Then Yuki notices that instead of the clothing they had that morning, their outfits are colour-coded. Then it turns out that this isn't their world, it's a Power Rangers-themed world.
- Again in episode 14, right down to them thinking they're back. Then they notice Caiden wearing glasses. It's a superhero world, and Caiden is Clark Kenting.
- In The Pajama Party Murders, Lola's disguise as Myrtle is revealed when someone points out that, after being called Uncle Cosmo's non-swearing, non-drinking missionary, she swore and took a swig of beer. From there, Pettibone runs down a list of clues that she was an impostor.
- Mary from The Children's Hour has convinced her grandmother that her two teachers are having a lesbian affair. Her grandmother is an influential person and within days all the students have been taken away by their parents. When Karen and Martha confront Mary's grandmother about her Malicious Slander they ask Mary to explain. Mary keeps to her lie however she says she saw the two doing things when she looked through the keyhole to Martha's door. The problem is, as Martha angrily points out, her door doesn't have a keyhole. Mary then switches her lie to having heard noises from Karen's room, however Karen's room is too far for anyone to have heard anything. When she can't figure out a new excuse Mary says she heard the rumor from a classmate, who heard it when the door was open. Mary blackmails Rosalie to get her to agree with the lie. Karen and Martha spot the thread, however everyone else believes the kids including Karen's fiance.
- In the beginning of Act 2 of Les Misérables, Javert disguises himself as a disgruntled ex-soldier who helps out the revolutionaries by acting as a spy and attempting to feed them false information. He's only found out when Gavroche reveals him as a cop. It's a wonder they didn't realize this earlier, as Eponine's panicked warning in The Robbery indicates that he's a quite well-known police officer, and he had interacted with three of the revolutionaries (Marius, Gavroche, and Eponine herself) before going undercover.
- National Theatre's 2014 production of Treasure Island:
- After Jim overhears the pirates plotting mutiny and reports it to Captain Smollett, Smollett nearly manages to maneuver the would-be mutineers into going ashore where they can be threatened with marooning, pretending that he's unaware of their treachery and offering them the first chance to set foot on dry land after the long voyage. Jim accidentally blows the gaff by making a comment about Long John Silver having been on the island before; Silver recalls that he's been careful never to mention that fact except to his fellow mutineers, realizing that this means Jim heard their conversation.
- Near the end, Trelawney tries to bluff the pirates by pretending to be the ghostly voice of the late Captain Flint. Armed with details about them supplied by Ben Gunn, he nearly pulls it off until Silver gets suspicious because "Flint" also knows the name of a mutineer who threw in with the pirates on this voyage and was never part of Flint's crew.
- Quite often in the Ace Attorney series. The defense attorney you're playing as needs to spot a contradiction in a witness testimony, present a theory, and back it up with evidence. Inevitably, this results in some Courtroom Antics to help catch the real criminal.
- During case 4 of the first Ace Attorney game, Phoenix Wright completely bluffs about how someone who had been shot could have carried a bullet from the scene of a crime...and, following it to its logical conclusion, helps prove it. To wit, the reason the bullet left the scene of the crime is because it hit a person. The only person present at the time had to be the culprit, because otherwise the other person would've been a witness and no witness came forward. Phoenix's suspect Manfred von Karma wouldn't have removed it because doing so would leave a paper trail, and if he was shot and didn't get surgery for it, the bullet should still be in his body. And if it's still in his body, it could be found with a metal detector... that Phoenix just happened to have on him, after using it previously. Naturally, the detector finds metal. When von Karma tries to protest that it didn't have to be the bullet, Phoenix notes that the police have the other bullet fired from the gun (the one that killed the victim), so the police could relatively easily compare ballistic markings.
- Ace Attorney Investigations 2 has a prisoner impersonating a security guard, only to get uncovered when his pet polar bear goes to him.
- Danganronpa: Trigger Happy Havoc: During the second scene of the game, the main character will note that Junko Enoshima looks different from her cover shots on magazines. This is then played off as the result of touch-ups and editing. Some of her dialogue during Free Time events also seems incongruous with her Ultimate Fashionista title. Chapter 6 reveals that the girl everyone met at the start was not Junko, but rather her [fraternal twin sister Mukuro Ikusaba, whose appearance is very similar, but with a few minor differences.
- In Cuanta Vida, Jeremy's green eyes and relaxed attitude tip off Jordi that he's dealing with Miguel, the RED spy.
- Shapeshifting fae Nutmeg from Dan and Mab's Furry Adventures pretends to be her husband, but Mab notices it because she didn't get the mannerism and hoof patterns right. Earlier, Jyrras uses a Magitek patch to pretend to be a cubi, but the disguise is pretty transparent to any real cubi (who are mind readers).
- Ennui GO!: In "Bop", Max starts to suspect that Kidz Bop is changing the lyrics of the songs they cover after listening to their bowdlerized version of "Fuck Tha Police", changed to "Trust The Police" — as he puts it, why would one of the most iconic gangsta rap groups sing about trusting the police? This leads to him and his friends seeking out the original song, and subsequently an Innocence Lost moment as they hear exactly how many swear words it has.
- Experience Boost: A part of Ohran's master plan was to ostracize Keran from the Realm of Quests Role Playing community by creating a hate blog filled with contempt for her fellow RP'ers. The blog went back months and was extremely dedicated and convincing, but it's all undone because he reblogged Keran's NOTP. Her roleplaying friends immediately realize they've been had, and come to help in the final battle.
- In this strip of Girl Genius, despite being in pain from having just swallowed a slaver wasp, Baron Klaus Wulfenbach is able to tell the person he's talking to isn't Agatha Heterodyne from her speech pattern, facial expression and body stance...and he finally recognizes she's (possessed by) Lucrezia from hearing her gloat.
- In El Goonish Shive, Tom's apology to Susan for missing her on a text messaging service tips Susan off that his intentions are not what they seem to be on the surface. He never asked whether or not she'd been online with that messenger service that night before apologizing for "missing" her online...and the messenger service has an "invisible" mode.
- The Order of the Stick:
- At one point Elan is replaced by his Evil Twin Nale, despite pretty much every thing he does being a possible invocation of this trope, all his teammates just chalk it up to Elan being, well, Elan. Until that is Nale changes out of the clothes he stole from Elan and takes a shower, at which point Belkar notices that "Elan" smells like Nale.
- Elan himself realizes that the team have been trapped in a Lotus-Eater Machine when Nale doesn't do anything to stop their parents remarrying. This realisation forces him to acknowledge that the remarriage doesn't make sense either, it's just what he'd want to happen.
- When Nale is later trapped in the same Lotus-Eater Machine, he snaps out of the illusion after realising that he had been monologuing to a captive Elan and Haley for over an hour and that they hadn't escaped his clutches in that time.
- Plane-spotters and birdwatchers do this in this xkcd strip, much to the frustration of a government cabal who now need to find a new disguise for their surveillance drones that doesn't have an obsessive spotting community. Turns out, there's a lot of people who like staring at the sky.
- At times, This Very Wiki. That "Super Secret Spoiler" wasn't so Super Secret when some extra white space where there shouldn't be popped up.
- Of course, due to the YMMV finagle, Super Secret Spoiler has been replaced with the normal spoiler.
- Averted deliberately by some clever tropers by using completely unnecessary spoiler space or adding innocuous information under spoilers in strategic places, and making spoilers longer . . . yes, longer than necessary so their length doesn't give anything away, such as the hidden text being the exact length of a character's name.
- Parodied in SF Debris's reviews of Star Trek: Voyager, in which it turns out that all the hallucinations and fantasies share one common flaw: Chakotay is too lifelike.
- Help Not Wanted: Ogrell Syn'Gorrsh speaks in Third-Person Person all the time, but in chapter 6, he tells the four goblin contractors, "I'm not a trustworthy person." They realize immediately that he's been lying about who he says he is, and it's revealed that he's a Serial Killer who pretends to act stupid so his victims will lower their guard around him.
- Loves Lost And Found: Maria is having dinner with a strigoi disguised as Hardestadt Delac. While in disguise, the strigoi calls Maria a woman, despite the fact that Maria is non-binary. They notice, and shortly afterwards, Maria kills the impostor.
- In the Search and Rescue Woods Creepypasta series, the protagonist hears a crying child, and goes to investigate. He then notices something wrong: the "crying" is the same sound over and over again, like a song on repeat. He wisely nopes out of there.
- In the When Posting Goes Wrong episode about the HIVLiving scandal from tumblr, a blogger named Ursula does this twice, which blows the whole thing open.
- First, the lie that led to them confronting the mod of the HIVLiving blog, "Israa", who claimed to be an HIV+ sex trafficiking survivor living in India. Ursula suspected Israa wasn't being honest about her background or HIV status, and did some digging—which found a post she put up asking for donations, ostensibly for medical reasons. The problem? The app she asked people to use to send her money only worked in America.
- This led to them confronting "Israa", who, after some prodding, admitted she was a college student named Alix living in the United States. However, she insisted Israa was real and that they were friends—and that she really was HIV+. Ursula, obviously, wasn't buying it, and continued to grill them for several hours, until finally catching Alix in a lie that they absolutely could not wiggle out of. Alix claimed to share a tumblr login with her roommate, which was already dubious at best, and then claimed her roommate didn't know she had HIV. Except, as Ursula points out, Alix frequently mentions her HIV status on her blog, including in her bio. If she and her roommate shared a login, there's no way she wouldn't have seen it.
Alix: oh shit
- Oxventure: In one episode, the Oxventurers are trying to find a werebear that has turned back into their human form after the Sun rose. Prudence and Corazon manage to spot that among the various townsfolk setting up work in the morning, the town blacksmith is nervously hammering away at a stone-cold forge.
- Cermit in the DevilArtemis universe gave himself away as having impersonated Kermit when he referred to their friend "Broly", not knowing that Kermit calls him Billy.
- The Vault: Mail Room Guy decides to taunt the other characters by mockingly reading their letters instead of being useful. As he does, multiple characters are shown visually reacting with confusion before they start pointing out multiple details he got wrong: He had stated one girl who had 6 siblings only had one sister (and had the incorrect name) and said Henry went to a university he didn't actually go to. This causes them to believe the letters are fake before Henry points out the names are just mixed up, wordlessly admitting to having noticed a different thread that a letter matching him was put under a different alias.
- There is a story of a British spy in occupied France during WWII who was exposed after ordering "cafe noir" (black coffee). A local would have known that, with milk being scarce, the "noir" was redundant as that was all there was on offer.
- Relatedly, hapless German and Russian spies in and after WWII ran into trouble when, if they made it to England, they ordered tea without milk whilst posing as working-class Brits. That would be seen as a little odd today. In the '40s and '50s, you may as well have turned up with a sandwich board saying, “I’m a spy.”
- There are also many, many cases of German spies being picked up almost immediately after arriving in Britain, largely because the various feuding Nazi organisations' information was woefully out of date. In particular, they never seemed to grasp the intricacies of the British rationing system (for example, trying to pay for meals in restaurants with food ration tickets).
- There were stories of British spies nearly getting caught for putting spoonfuls of soup straight into their mouths (in parts of France at the time it was customary for women to sip from the side of the spoon) and, of course, forgetting that the French drive on the right side of the road. Similar stories include (in this case, US) spies outed when they switch their fork from the left to right hand to eat, rather than the Continental style which keeps the fork in the left hand.
- There's a true story about a suspected German spy being questioned for several days. The interrogators had their suspicions, but there was nothing definite, so they let the man go. He blurted out, "Danke..." The story is also told the other way around - an American spy, after days of interrogation, being told in German, "Well, seems you're not a spy after all, you can go" to which they replied - in English - "Thank you!"
- And another one about two German spies entering a bar in USA and making the following exchange with the bartender:
Spy: Two martinis, please.
Bartender: Dry? (homophonic to German "drei" - "three")
Spy: Nein, zwei!
- And another one about German infiltrators during Battle of the Bulge:
- Whenever German spies infiltrated the Soviet army, they usually had perfect and impeccable histories, accents and documents. But the staples on their documents were shiny stainless steel — the USSR was still stuck with iron staples, which rusted.
- Germans-dressed-as-Americans during the Battle of the Bulge were given away due to their forgers correcting an intentional spelling error on the standard GI identity cards.
- American spies were supposedly outed by the Germans for crossing their legs the American way, by resting their ankle on the opposite knee. Europeans cross their legs by resting the back of one knee over the other.
- Many a German spy was outed in World War II for failing to comprehend the mind-numbingly complicated British Imperial monetary system. Until 1971, the complex coinage, nonsensical divisions of currency (240 pennies to a pound, quarters of pennies, etc.), and numerous slang terms for the various denominations meant that merely counting change was extremely difficult for foreigners. This also nearly tripped up a sympathetic spy, Juan Pujol Garcia, a Spaniard who took it upon himself to feed false information to the Germans (and who was later inducted into the British spy network). Instead of trying to add up the totals on the fake expense accounts from his fake agents in the UK, he simply sent out itemized lists of things (like information) they had supposedly bought.
- One of Pujol's fake reports claimed that in Glasgow (which Pujol never visited, being based in Lisbon during this time) there were men who "will do anything for a liter of wine". Not only do Scots mostly drink ale or whisky, they use the Imperial system of measurements, not the metric. Somehow the Nazis never noticed this. (Then again, selling wine in metric measurements is quite common, even in the US.)
- Legend has it that the "Romeo" spies used by East German spymaster Markus Wolf to prey on lonely West German women who had access to relevant information eventually got compromised by German train conductors who noticed their totally unfashionable haircuts as soon as they entered West Germany.
- Roger Bushell, the mastermind behind the famous WWII "Great Escape", and his traveling companion were captured in this way. The pair were pretending to be French, but an officer became suspicious and fired off a sudden question in English. Bushell's companion (who was actually from France and whose native tongue was French, but had grown accustomed to speaking English with his fellow POWs in the camp) instinctively responded in English as well. A fictionalized but relatively close version of the incident is depicted in the film version of the story (see entry under "Film").
- Shibboleths are words that rely heavily on language-specific pronunciation. These have often been used to distinguish infiltrators from native speakers. Of course, they are useless against traitors.
- The term itself comes from The Bible, in an account where, during a conflict between the Gileadites and Ephraimites in one of Israel's pre-monarchy civil wars, the Gileadites identified fleeing Ephraimites by their pronunciation of the word shibbólet (שִׁבֹּלֶת) (meaning "ear" in the sense of "ear of corn", i.e., "the part of a cereal plant that contains the grains"), which would have been pronounced sibbolet in the Ephraimite dialect.
- The Dutch used the name of the port town Scheveningen* to unmask Germans.
- The Parsley Massacre. During the presidency of Rafael Trujillo, Dominican soldiers would have people suspected of being Haitian Creoles say the word "perejil" (the Spanish word for parsley); Haitian Creoles struggled to pronounce it correctly and were killed.
- In 1302, during the Bruges Matins the Flemish used the phrase "Schild en Vriend" ("Shield and friend") to identify and murder French sympathizers.
- In 1312, soldiers of the Polish duke (later king) Władysław Łokietek rooted out foreign members of a burgher revolt in Kraków by forcing every interrogated citizen to say "soczewica koło miele młyn" - a nonsensicalnote string of Polish words, which were difficult to pronounce properly in fast succession by anyone not intimately familiar with the language. The city was summarily punished for the revolt anyway, but the foreigners identified this way received some extra attention.
- Another WWII story tells of a soldier who tried very hard to pass as a civilian while in Europe. He got new clothes, a new hairstyle, learned the local language, etc. One day, while waiting on the curb with two bags of groceries in his hand, a German walks up and basically greets him with, "Hi, Soldier!" The reason? He had two bags of groceries in one hand, because the military ingrains the habit of keeping your other hand free to salute.
- During WWII, German spies would often try to infiltrate Britain via neutral Spain, posing as Swiss businessmen in order to travel to Spain. The problem was, German standards of discipline were too high. German spies were under standing orders to stay sober, not frequent brothels, and absolutely not to deal on the black market, whereas given how expensive and heavily taxed cigarettes, sugar and other luxuries had become in Switzerland during World War Two, MI6 could tell with a good degree of accuracy who were real Swiss businessmen and who were spies simply by offering to sell them some cigarettes or nylons on the sly. At one point, the German intelligence services were receiving hundreds of reports from agents "in London" per month, all coming from agents caught and turned in Spain, writing fake reports from Barcelona using "The A-Z of London" and copies of The Times to keep their fake reports realistic. All because the spies were more honest in their dealings than the civilians.
- An interview with early female CIA operatives suggested that women at the CIA were better at this than men. One now senior analyst stated, "You could always tell them by their socks." Another pointed out that regular Russian embassy employees always had cheap shoes, but the intelligence agents had very nice shoes.
- This trope is Played for Laughs (in tandem with what appears to be either Political Overcorrectness or just Eagleland) in this Cold War-era joke:
An elite American spy, after extensive and grueling preparation, is parachuted over Siberia in the dead of winter. He quickly gets rid of the parachute, dons local garment and starts to wade through the snow. After a lengthy march he finds a lonely hut, knocks at the door, and is allowed to enter by an elderly lady living in it. She invites him for a snack, and after some time asks:
Lady: You're an American, aren't you?
Spy: But Granny note , how could you?! Don't I speak like a Russian?
Lady: You do!
Spy: Am I not dressed like a Russian?
Lady: You are!
Spy: Don't I drink like a Russian?
Lady: Oh, yes, you do indeed!
Spy: Then why are you saying such things?!
Lady: You see, son...it's because we have no Negroes.
- This is how the Allies spotted Ernst Kaltenbrunner, the Nazi General in the SS. Kaltenbrunner gave himself up claiming to be a doctor and offering a false name. However, his mistress spotted him, and by chance occurrence, she called out his name and rushed to hug him. On 12 May 1945, this action tipped off the Allied troops, resulting in his capture, trial, and execution.
- There are quite a few videos on YouTube of Phony Veterans like this one getting called out by actual soldiers for claiming to be part of nonexistent battalions or wearing medals improperly.
- The English highwayman Dick Turpin was reportedly caught this way after fleeing to Yorkshire and changing his name. Dick wrote a letter to his family, but he'd been taught to write by a postman. So when that postman received the letter and realized 'John Palmer' had exactly the same handwriting as a notorious armed robber, the trail led back to Dick and he was arrested and executed.
- One of the things that tipped off America that something was up in Cuba, ultimately leading to the missile crisis in 1962, was that Cuba started building soccer fields in high numbers. At the time, the sport of choice in Cuba was baseball - the Soviet Union preferred soccer.
- A story goes that some WW2 American sentries used a line from the third stanza of "The Star-Spangled Banner" as a password, specifically "the terror of flight". Anyone answering "the gloom of the grave" was immediately arrested as a spy, because what real American knows the entire national anthem by heart?
- Numerous cop shows have had a moment where one of the detectives mentions seeing or hearing some ostensibly minor thing that tips them off to the fact that someone is lying. Such as spotting a large bouquet of flowers and realizing that a man isn't being truthful about his supposedly happy marriage—"There's "Honey, I was just thinking about you" flowers and "Honey, I'm sorry, I fucked up" flowers. These were "I fucked up" flowers.", or how a lack of stolen things negates someone's claim that a murder happened in the course of a robbery, etc.
- During the collapse of Nazi Germany, SS-Reichsführer Heinrich Himmler, who was hunted by both the Allies for his crimes and by the Germans for attempting to negotiate peace behind Hitler's back, attempted to evade capture by posing as a common soldier. He had an extensive set of fake identity documents prepared for this. However, the British became suspicious of a man whose papers were entirely in order, and noticed that they included a stamp used by fleeing SS members. This led to his detainment and suicide in a POW camp.
- In 1972, Klaus Altmann, a German living in Bolivia, was interviewed by French journalists after being accused by Nazi Hunter Beate Klarsfeld of being notorious war criminal Klaus Barbie. Altmann vehemently denied being Barbie until the journalist asked him if he'd ever been to Lyon (where Barbie had been head of the Gestapo). Altmann replied that he hadn't... except that the question had been asked in French, a language "Klaus Altmann" would not have spoken, but that Barbie did speak. He tried to cover himself by claiming to have been to Versailles during the war, but the damage was done and his identity was confirmed soon after, allowing him to eventually be extradited and sentenced to life imprisonment for war crimes.
- What really exposed the American Mafia to outside scrutiny was when a suspicious New York state trooper noticed the expensive cars bearing out-of-state plates parked on the ranch of a businessman who had ties to organized crime and called in reinforcements. The attendees tried to flee the scene when they realized what was happening, but more than sixty mobsters were arrested. While their convictions were overturned on appeal because there was no evidence of wrongdoing prior to the Apalachin Meeting being broken up, it was very damning for a criminal organization that valued secrecy above all. FBI director J. Edgar Hoover, who had long denied that anything like The Mafia existed, ate his words and set up the Top Hoodlum Program to target the mob bosses.