[[quoteright:350:http://static.tvtropes.org/pmwiki/pub/images/fire-extinguisher.jpg]]
[[caption-width-right:350:[-Move along, nothing to see here. Hey, ''waaaaait a minute...''-] ]]

->"''A web of lies can unravel with the lightest touch of the truth!''"
-->-- '''Othar Tryggvassen''' (GentlemanAdventurer!), ''Webcomic/GirlGenius''

This is when an impersonator has an almost perfect disguise, only to ruin it with a seemingly inconspicuous mistake. Perhaps the imposter blurted out [[OutOfCharacterAlert something out-of-character]] (or a [[SomethingOnlyTheyWouldSay personal catchphrase]]), or accidentally revealed that [[ASinisterClue they're left-handed]]. %% *NOT* IAmNotLeftHanded.
This can more easily occur during a round of BluffTheImpostor, and can be a sub-trope of SpotTheImpostor except that their target generally has no ''advance'' knowledge that there is an imposter in the first place.

Related to PullTheThread as this is often what leads to it. Compare SayingTooMuch, a more incriminating version. See also ConvictionByCounterfactualClue, when this gets even more unrealistic. Often overlaps with ImposterForgotOneDetail. Compare ForWantOfANail.

When applied to a dream test, it's AGlitchInTheMatrix.

----
!!Examples:

[[foldercontrol]]%%Do not remove the folders, they are the standard.

[[folder:Advertising]]
* PlayedForLaughs in a Bush's Baked Beans Ad:
-->'''Talking dog dressed as ghost:''' How did you know it was me?\\
'''Human:''' Grandpa didn't have a tail.
[[/folder]]

[[folder:Anime & Manga]]
* ''{{Claymore}}'': Raki is told that a Claymore is looking for him and is in the nearby forest. When they talk Raki points out, correctly, that Claymores never refer to themselves as Claymores[[note]] the name was given to them by regular humans[[/note]]. TheReveal is that it was a Yoma impersonating a Claymore to take him hostage in order to draw Clare out and kill her.
* Twice in ''Manga/FullmetalAlchemist''. The first time is where Maes Hughes catches out the shape-shifter Envy because he forgot a [[BeautyMark mole]] when he took the appearance of Maria Ross. The second time is where Riza catches Envy out again when he took the appearance of [[spoiler:Roy Mustang, claiming that they were on a FirstNameBasis when alone]], but then admitting that she [[BluffTheImposter lied when he panicked]] - played with in this case, in that she ''fakes'' this trope [[YouJustToldMe in order to expose him]].
* ''Manga/MahouSenseiNegima'': The Takamichi illusion during the BadFuture RescueArc.
** Also done humorously, when Negi realizes that the Shizuna-sensei he's speaking to is an impostor because of a MarshmallowHell. Turns out that the impostor (Asakura) didn't quite measure up to Shizuna's bust size.
* ''Manga/{{Naruto}}'': Sasuke recognizes a Naruto impostor because he doesn't have a cut on his cheek. (Naruto received it before they entered the Forest of Death.)
** After this incident, the team decides to have a secret password, so if they split up and someone comes back, they can confirm their identity via the password. Naruto promptly is separated from the group, rejoins them, and recites the password correctly. Sasuke attacks, knowing that the real Naruto would never remember the intentionally long and complicated password, especially on his first try.
** Early in the series, [[TagalongKid Kohonamaru]] is spotted trying to trail Naruto, as he is holding his fence-colored camouflage in the wrong direction.
* The ''Franchise/LupinIII'' franchise has Lupin saying "Tot-san" ("Pops" or "Old Man" in English dubs). In the ''Anime/LupinIIIVSDetectiveConan'' MadeForTVMovie, Zenigata identifies the disguised Lupin when Lupin calls him by his nickname.
* In the Alabasta arc in ''Manga/OnePiece'', the straw hats agree on a secret sign to distinguish each other if shape shifter Mr 2/Bon Clay assumes their shape. We are shown them wearing a bandage on their arm [[spoiler:but the real sign is [[UnspokenPlanGuarantee black X drawn on the arm under the bandage, which Mr 2/Bon Clay misses when he pretends to be Usopp]]]].
* In ''Anime/{{Monster}}'', Tenma is initially suspicious with two "friendly" helpers being accomplices of Johan and has his suspicions confirmed when one of them refers to him as "doctor" -- something he never told them about.
* ''AxisPowersHetalia'': England runs afoul this trope in the Can't Escape from Italy strips. His Italian disguise is eventually perfected with the help of a spy, right down to taping a curl to the side of his head, but Germany sniffs him out when England stands posturing instead of hitting on the attractive women who pass by.
** Germany himself is outed by France when he starts mashing his potatoes up instead of slicing them.
* In ''Anime/SailorMoon R'' Creator/DiC dub, one of the MonstersOfTheWeek disguise themselves as Chibi-Usa's mom and calls to her using the name Small Lady. Chibi-Usa is at first fooled, happy to see her momma again, and runs to hug her. She then suddenly stops and asked the monster who they were. When the monster replies that she was her mother, Chibi-Usa say she's lying because her mother never calls her Small Lady, her royal title. In the original Japanese the monster calls her Rabbit (which is what the Dark Moon minions had been calling her), because Small Lady is part of her actual name: Usagi Small Lady Serenity.
* Done in {{Alive The Final Evolution}}, where one of {{Big Bad}} Katsumata's Comrades was a female {{Body Surf}}er named Han sent to infiltrate the American military by entering the body of one of [[SergeantRock James McPherson]]'s subordinates to locate the {{Mineral MacGuffin}} "the heart of Akuro". Despite possessing the memories of his host body, Han is eventually shot in the heart by McPherson when he notices that his subordinate keeps his nails unusually clean and manicured.
[[/folder]]

[[folder:Comic Books]]
* In the Franchise/{{Batman}} crossover ''[[Comicbook/BatmanNoMansLand No Man's Land]]'', Gotham City is hit with a cataclysmic earthquake and quarantined. Batman is in the city trying to clean it up, having sent Superman away. But Superman has come back as Clark Kent, trying to help people on a different scale. He tells Batman that he did his best to rough up his clothes so as not to stand out and asks Batman how he did. Batman looks him over and says "The toes of your shoes are scuffed, but you forgot to scuff the heels. Your shirt is dirty but bears no evidence of sweat or epidermal oil secretion stains. And no one around here has smelled of deodorant soap and laundry detergent for over five months." Clark asks how he looks to someone who isn't the world's greatest detective. Batman says he looks fine.
** Previously in the inaugural arc of GrantMorrison's JusticeLeague, Batman deduces the true nature of the Hyperclan this way. Strength, speed, shapeshifting, aversion of fire [[spoiler: They're Martians]]. They got away with it as long as they did with each one emphasizing a different power from the common power set.
* The Franchise/{{Spider-Man}} villain Chameleon is caught out like this twice when he goes after Mary Jane and Aunt May, both of whom are easily able to tell that he's not the real Peter Parker through clues like this, and are thus able to administer particularly [[SugarWiki/MomentOfAwesome awesome]] kicking of his ass as a result.
* {{Deadpool}} got a memory/flashback version in ComicBook/CableAndDeadpool-when T-Ray showed Deadpool the moment when Deadpool "tried to kill him and then stole his identity" in an earlier series, Deadpool was already in costume. When Cable fixed Deadpool's memories later, Deadpool remembered that he only got the costume after Weapon X, which he only joined because his cancer made him leave the army-
---> Deadpool: Which I signed up for as ''Wade Winston Wilson!''
** This gets [[FlipFlopOfGod retconned in and out]] DependingOnTheWriter, though fans typically take this as proof that Deadpool is Wade and T-Ray is not.
* In one comic, Franchise/{{Tintin}} disguised himself, but failed to disguise his dog.
* ComicBook/LuckyLuke: in the comic "The Singing Wire", Lucky Luke finds out that the Indian in their team is in fact not a real Indian but a disguised crook who had been sabotaging their attempts to build a telegraph wire, because he smells like shaving cream and Indians don't have beards.
* ''GIJoe'' villain Zartan uses [[VoluntaryShapeshifting several]] [[MasterofDisguise different]] [[HolographicDisguise methods]] to make himself into the perfect infiltrator, but is prone to missing details. Storm Shadow foils one disguise--a moped-riding priest--by recalling how Zartan drives motorcycles.
* When trapped in a LotusEaterMachine in the second story-arc of ''ComicBook/MyLittlePonyFriendshipIsMagicIDW'', Spike realizes it's not real when Rarity tells him to forget everyone else and serve only her, and doesn't fully appreciate the significance of the Fire Ruby he brought with him (it was previously a gift he gave to her).
* In Udon's ''[[Comicbook/{{Streetfighter}} Street Fighter II]]'' series, Guile compliments his secretary Gibson on her new contact lenses. When Gibson becomes confused by this statement, he mentions that she usually wears glasses. Later on, its revealed that the woman he was talking to was actually Juni, one of M. Bison's agents, and that the real Gibson was left BoundAndGagged in her own kitchen.
[[/folder]]

[[folder:Fanfiction]]
* During an escape in ''[[http://www.fanfiction.net/s/7728075/12/Of-Slaves-and-Sorrow Of Slaves and Sorrow]]'', [[{{Series/Merlin}} Arthur]] realizes Merlin was replaced with an enemy's illusion when [[ServileSnarker Merlin]] addresses him as "your highness" and "your majesty" with complete sincerity.
* ''FanFic/FriendshipIsAura'': Lucario is able to deduce that [[spoiler: "Celestia"]] is an imposter (actually [[spoiler: Chrysalis]]) by [[AwesomenessByAnalysis pointing out]] that she doesn't know who he is when she should, doesn't know the names of her own Guards despite socializing with them, doesn't know about her own diary (which he found earlier), and most importantly, doesn't know the nickname she [[spoiler: and Luna]] called her mother.
* In ''[[http://archiveofourown.org/works/237304/chapters/363777 The Big Reveal]]'', [[Creator/AnthonyHead Tony]] thinks there's something off about [[ColinMorgan Colin]] when he starts constantly speaking with an English accent when his normal one is Irish. He eventually suspects Colin's a different person when he asks for meat and cheese for dinner when he's supposed to be vegetarian and lactose intolerant.
* In ''FanFic/DiariesOfAMadman'', readers themselves have to do this in order to [[spoiler: realise Discord is lying to them and an UnreliableNarrator]].
* In ''FanFic/DirtySympathy'' Phoenix realizes that [[spoiler: Shadi Enigmar's]] death by bludgeoning is too spontaneous and physical for Kristoph to have done it as [[spoiler: Kristoph is a very patient poisoner.]]
* In ''FanFic/TheFifthAct'', Genesis realizes that [[PeggySue Cloud's]] story of being the younger Cloud's paternal uncle has one flaw in it: the older Cloud and younger Cloud have the same surname, when Strife is Cloud's mother's surname.
* ''[[https://www.fanfiction.net/s/4302297/12/In-His-Shoes In His Shoes]]'' has Iroh realize that someone else has taken over Zuko's body when [[spoiler: he isn't even tempted to join Azula and regain his honor. He's wrong about who it really is though- Iroh believes it's some kind of spirit that took over when Zuko died earlier in the story. Zuko and Sokka are actually in the middle of a FreakyFridayFlip]].
[[/folder]]

[[folder:Films -- Animated]]
* In the climactic battle in WesternAnimation/{{Megamind}}, Titan realizes that Metroman is actually [[spoiler:Megamind in disguise due to the distinctive way he mispronounces Metro City]].
[[/folder]]

[[folder:Films -- Live-Action]]
* ''Film/HarryPotter and the Goblet of Fire'': Barty Crouch Jr. used a Polyjuice potion to disguise himself as Mad-eye Moody. However, Barty Crouch Sr. spotted "Moody" doing Junior's signature tongue-flick, which gave him away. Note this was only done in the film version.
* In ''Film/X2XMenUnited'', Wolverine figures out Mystique isn't Jean because she has the scars where he stabbed her in the previous movie. Later in the same movie, Stryker takes one look at Mystique from across a large room and knows she's not Wolverine simply saying "I know my own work".
** In the novelization, it's her smell that alerts Wolverine even before he exposes the scars, but he plays along to try and find out what she wants.
* Zigzagged in ''Film/{{Scream 1996}}'': Billy Loomis, who has been suspected of being the killer for the entire movie, and even taken into custody at one point (although he was later cleared) is asked by his girlfriend Sidney who he called with his "one phone call" when he was arrested. He tells her that he called his father, but Sidney points out that the station called his father. [[spoiler: Not thirty seconds later, he is killed, exonerating him from suspicion. However, it turns out that he actually ''was'' the killer (or one of them anyway) and had faked his death, and the thread Sidney had spotted earlier was probably correct.]]
* In the 1974 film ''Film/GodzillaVSMechagodzilla'', the human protagonists begin to suspect that the disguised Mechagodzilla is a fake when it attacks Anguirus... something the REAL Godzilla wouldn't do since he and Anguirus are allies. Their suspicions are confirmed when the original Godzilla shows up to do battle against Mechagodzilla for the first time.
** Anguirus also immediately notices that the disguised Mechagodzilla is a fake because it doesn't sound or act like the Godzilla he knows.
* In ''Film/InglouriousBasterds'', [[spoiler:a Nazi notices an impostor by which fingers he holds up when he asks for three of something: he does "index-middle-ring" and not "thumb-index-middle", as Germans usually do it.]] An honest mistake usually, but this is Creator/QuentinTarantino, so there's a bloodbath over it.
** He already noticed the impostor by his unplaceable accent (the Nazi is an accent buff of Henry-Higgins-like extremes), the drinks gesture only confirmed his suspicions. In fact, he intentionally ''manipulates'' the spy into placing that order.
* In ''Film/TheGreatEscape'', this is how Bartlett and [=MacDonald=] are captured. A Gestapo agent asks to see their identification and asks them questions in German. When he is finished, he says "good luck" to them in English, and [=MacDonald=] blunders by replying in English.
** Furthermore, this is harsh irony, because [=MacDonald=] had previously admonished one of his men for falling for the same trick when [=MacDonald=] plays it on him. Apparently he wasn't kidding about how important it is for your facade to be 100% ''perfect''.
* In ''Film/{{Inception}}'' there's an example of ''literally'' spotting the thread, when [[spoiler:Saito notices that the carpet in a room he's in is made of the wrong fibre, tipping him off that he's still in a dream.]]
* In ''Film/AttackOfTheKillerTomatoes'', "Master of Disguise" Sam Smith blows his cover after infiltrating the tomato camp, by asking for ketchup to put on the arm he's about to eat.
* The 2003 remake of ''TheItalianJob'' has this: Steve realizes Stella is John's daughter because of a phrase she uses, that he's never heard from anyone one else but John.
-->'''Steve:''' Still don't trust me?\\
'''Stella:''' I trust everyone. I just don't trust the devil inside them.
* In the documentary ''Catfish'', Nev begins to realize [[spoiler:Megan is not real]] because he finds out she took other people's songs and said they were hers.
* The dramatic thrust of ''Film/SuckerPunch'' is almost entirely dependent on whether or not the audience can do this. There's a really big clue, though, just before TheReveal that a twist is coming because a character's wearing a dress she shouldn't have.
* In ''Film/StarTrekIVTheVoyageHome'', Kirk and party are within seconds of successfully bluffing their way out of the hospital with the wounded Chekov in tow when this exchange occurs.
-->'''Guard:''' How's the patient, Doctor?\\
'''Kirk:''' He's going to make it.\\
'''Guard:''' He? They went in with a ''she''.\\
'''Kirk:''' (to himself) One little mistake... [they run]
* In ''{{Paycheck}}'', Jennings suspects that the Rachel he meets in the cafe is an impostor when he sees a colored contact lens shift out of position while they kiss.
* in ''{{Stalag 17}}'', the Germans have planted a spy, Price, in a POW camp posing as an American flier from Cleveland. Sefton (William Holden) suspects the ruse, and in the climactic scene confronts Price:
-->'''Sefton:''' When was Pearl Harbor, Price, or don't you know that?\\
'''Price:''' December 7th, '41.\\
'''Sefton:''' What time?\\
'''Price:''' 6:00. I was having dinner.\\
'''Sefton:''' 6:00 in Berlin. [To the other barrack members:] They were having lunch in Cleveland.
* Xander Cage spotting the fake diner hold-up in ''Film/XXx'':
-->'''Xander:''' You know, you almost had me going there for a while. I was a bit groggy before, then I started noticing things. Like, you got a stockbroker over here, all dressed up reading the Financial Times on a Sunday morning when the market's closed. Unlikely, but okay, I can go with that. I can even go with the stick-up man packing a cop-issue Beretta. But you want to know where you blew it? With the waitress. My aunt was in the restaurant business all her life. There's no way in hell a career waitress comes to work in high heels. She'd have blisters the size of pancakes before lunch. And if she ain't real, then this whole thing ain't real. That's how I knew this bozo over here wouldn't get a shot off even if we waited till St. Patrick's Day. Because there's nothing but blanks in these guns. Oh, and no offense, but their performances were terrible.
** Subverted in the next scene when he assumes they are still playing him after dumping him in Columbian drug cartel territory. He figures it out just before Danny Trejo goes to work on him with a machete.
* In ''Film/{{Dredd}}'', Judge Dredd trips up a corrupt Judge on Ma-Ma's payroll this way. When the Judge states that he was responding to a call for backup, Dredd points out that the Judge didn't ask about the rookie Anderson.
* In ''Film/{{Wishmaster}}'', Alex eventually catches on that the Djinn disguised himself as one of her friends. The Djinn manages to explain a SayingTooMuch slip-up, but his antagonistic behavior and constant offers to do something for her (thereby invoking a wish) give him away.
* In ''Film/CaptainAmericaTheFirstAvenger'', Steve Rogers wakes up in a 1940's-era room. A woman wearing a period-specific military room enters to greet him. Rogers catches on that something is amiss when he remembers being at a baseball game that was seemingly broadcast live over a radio in the same room.
** ''Film/CaptainAmericaTheWinterSoldier'': Steve notices an ammo bunker at what's left of the decommissioned Camp Lehigh that's way too close to the barracks for either safety or Army regulations. It turns out to be [[spoiler: the secret base holding Zola's uploaded brain]].
* In ''Film/GhostShip'', Epps catches on that the villain has killed and impersonated her last remaining crewmate when he doesn't ask her what happened to the second-to-last one, who's been killed in the interim. He drops the act and admits he already knows because he killed the other one too.
* In ''[[Franchise/DieHard Die Hard with a Vengeance]]'', [=McClane=] gets in an elevator with several terrorists disguised as cops and security. He notices they're using terminology incorrectly, referring to the elevator as a "lift" (a European/British term) and a weather report as raining "dogs and cats" (the wrong order). [=McClane=] is visibly suspicious, but what confirms it is when he spots "Detective Otto" is wearing a [[ChekhovsGun police badge]] that belongs to a friend of his. [=McClane=] being [[CowboyCop McClane]], things inevitably get violent.
* In ''[[Film/MissionImpossible Mission Impossible: Ghost Protocol]]'', murderous thief Sabine is [[GenreSavvy canny enough]] to know that IMF is after the intelligence she stole (killing IMF Agent Hanaway to do so), and realizes that Ethan and Brandt are IMF agents when she sees Brandt's hi-tech contact lens, which Hanaway had been wearing when she killed him at the start of the film.
* In ''Film/{{Godzilla 2014}}'', after breaking back into old Janjira, after it was quarantined due to a reactor meltdown, Joe Brody realizes the place isn't radioactive when he spots three dogs chasing each other, when they should be dead from radiation. A quick check of his Geiger counter confirms his suspicions.
[[/folder]]

[[folder:Literature]]
* In ''Literature/{{Animorphs}}'', when Jake is infested with a Yeerk (mind-controlling slug), the GenreSavvy alien Ax immediately spots that something is wrong from his unusually vacant expression (it helps that just before that Jake fell head first into a pool full of Yeerks so the suspicion was not far-fetched). The Yeerk blows the disguise completely when Ax makes skin contact with Jake’s body, which revolts the Yeerk beyond limits and causes him to scream at Ax in rage. [[OutOfCharacterAlert "GET YOUR HANDS OFF ME, ANDALITE FILTH!"]].
** Gets better. The other Animorphs 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 in the way, the other Animorphs locked him up, knowing with 100% surety that he was a Controller.
* ''Literature/HarryPotterAndTheDeathlyHallows'': At Fleur and Bill's wedding, Harry uses the Polyjuice Potion to disguise himself. It doesn't fool [[{{Cloudcuckoolander}} Luna Lovegood]], who recognizes him from his expression.
** Harry and Ron make some errors when they turn into Crabbe and Goyle in ''[[Literature/HarryPotterAndTheChamberOfSecrets 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 ''[[Literature/HarryPotterAndTheGobletOfFire Goblet of Fire]]'', Dumbledore realized that [[spoiler:something was up with "Moody" when he removed Harry from Dumbledore's sight in the midst of all the chaos surrounding Cedric's death.]]
** Also in ''Hallows'', the Death Eaters realize which one of the several Harrys are the real one when he uses his "signature" spell, Expelliarmus.
* ''TheAdventuresOfHuckleberryFinn'': Huck goes in disguise as a girl at one point; the woman he's talking to gets suspicious when he can't thread a needle, and he is found out because of the way he catches a ball of yarn tossed to him. In short, Huck didn't do his research.
** If memory serves, he could thread a needle, only he was putting a needle on a thread, not the thread into the needle like any girl would do. Makes less sense, though.
*** The real giveaway was how he caught the yarn: A girl would open her legs, to catch the ball on her skirt, but a boy would close his legs to catch it on his lap. Oh, and anyone who'd ever done sewing would know that putting the thread into the needle is much, much easier than putting the needle onto the thread.
* In the book ''[[Literature/HonorHarrington Echoes of Honor]]'', a Haven admiral notices that the planet Hell 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 StateSec uses computer imagery to disguise the appearance of the StateSec crew 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 Andermani know her personally, at the time she was thought dead after apparently being executed.
* In ''Faded Steel Heat'', Garrett is impersonated by [[spoiler:a shape shifter]], who visits Morley Dotes's 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. If I recall correctly, flushing them out was actually one of the reasons for feeding him that meal.
* 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 ''TheBookOfTheNewSun'', Severian spots a ''literal'' thread: [[spoiler:a ribbon sticking out from the side of the head of the mysterious soldier he battles at the Sanguinary Fields, which tips him off that it's really Agilus in disguise.]]
** Not quite: 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 UnreliableNarrator, because he's not a 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 [[spoiler: being manipulated by time-traveling aliens?]] Pull the thread..
* In ''Literature/DoubleStar'' by Creator/RobertAHeinlein, 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? Similarly to the Garrett Files example, 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..."
* {{Literature/Eisenhorn}}:
-->'''Eisenhorn''': You could talk like [[spoiler:Nayl]] and use Glossia [[spoiler:thanks to Jekud]], but [[spoiler:Jekud]] didn't know what [[spoiler:Nayl]] knew. We fought Sadia on Lethe Eleven, not Eechan.
* In the GeorgetteHeyer novel ''False Colours'', Kit is caught in a TwinSwitch when he opens a snuffbox the wrong way.
* In KeithLaumer'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.
* ''Literature/GalaxyOfFear'' 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 literal DarthVaderClone. 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.
* MaximumRide's clone might have been a more successful replacement if she had known that the blind guy was the TeamChef, but given that [[spoiler: the flock's youngest member is a freakin' ''[[{{Telepathy}} mind reader]]'' the chances are slim]].
* In ''Literature/{{Ghost}}'', when looking for [[spoiler: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.
* Aly does her best to avoid mistakes of this kind in ''Literature/DaughterOfTheLioness'' 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.
[[/folder]]

[[folder:Live-Action TV]]
* ''Series/{{Blackadder}} II'': Prince Ludwig, master of disguise, tries to sneak into Queen Elizabeth's fancy dress party disguised as Nursie in a cow costume, because Nursie ''always'' goes to fancy dress parties as a cow. But he forgot one little thing (or in this case didn't know about it): Nursie is insane and her cow costume looks nothing like a cow. His disguise was too good.
** An earlier episode had Blackadder and company trying to impersonate a man who they prematurely executed for a last meeting with his wife, and needing to preempt various threads that she fortunately mentions just before meeting him; they almost stumble at the fact that Blackadder pretends to be missing more arm than the man actually lost, but covers it up by claiming to have gotten in a fight with another prisoner. At the end of the episode, he has to do the same thing with the Queen (who happens to be talking about how unexpected it was that the man was a traitor, considering his various debilities...).
** ''Series/{{Blackadder}} Goes Forth". Similarly, Blackadder manages to discover an undercover German agent when she fails to spot that out of Oxford, Cambridge and Hull; one of them is NOT a great university. (As an in-joke, Rowan Atkinson, who plays Blackadder, attended Oxford, whereas Stephen Fry, who plays Melchett, attended Cambridge).
---> '''General Melchett''': Quite right, Oxford is a complete dump.
* ''Series/RedDwarf'': TWO examples in the season 6 opener "Psirens"; the first being when a psiren masquerading as Kryten [[OutOfCharacterAlert refers to Lister as 'Dave']], and the second when a psiren masquerading as Lister gives himself away [[ItMakesSenseInContext by playing the guitar ''too'' well.]]
* ''Series/{{Leverage}}'': In ''The Second David Job'', Eliot poses as an antiques expert and takes Nate's ex-wife out for coffee while the rest of the gang watch from inside a van. His ex-wife appears to buy it until she looks at one Eliot's buttons and points out that she bought that exact camera for Nate on his birthday. Cut to her walking up to Nate's van and demanding an explanation.
** She screwed with Nate's head a little bit first. Justifiably.
* ''Series/{{Batman}}'': When the police chief wipes his forehead with the wrong hand, this tips off Batman that it's actually the villain False Face in a LatexPerfection mask.
* ''Series/StarTrekDeepSpaceNine'' subverts it: Keiko O'Brien becomes suspicious of the recording provided of her husband's death, as the time stamp shows it's late in the day and computer analysis shows that the cup he's drinking from (shortly before an explosion) contains coffee, and she ''knows'' her husband never drinks coffee in the afternoon as it keeps him awake at night. Sure enough, the video has been faked, and her husband is rescued. He finally makes it back home that night, and says "boy, that was a long day. I need a cup of coffee!".
* ''Series/StarTrekTheNextGeneration'' played it straight in "Datalore", when Lore tries to pass himself off as Data and slips up by mentioning his "off switch". Data had previously sworn Dr. Crusher to secrecy over it. Other smaller things, such as his use of contractions (whereas Data does not) tipped them off to the charade.
** In the episode, ''Future Imperfect'', Riker is in a simulation designed to make him feel it is 16 years in the future and he is captain. Among the threads he spots: The computer takes forever to respond to requests, Geordi takes over 30 hours to run a level one diagnostic on this problem ("You're incapable of this level of incompetence!"), Data cannot do trillions of calculations per second and uses contractions (but denies it immediately afterward), and finally when he sees pictures of his dead wife it turns out to be a non-existant girl he met on the holodeck (they scanned his memory for a woman he had loved and lost but whose actual death he had never grieved). He's suspicious that something is ''off'' but isn't sure what until he sees the pictures, then realizes all the other threads confirm he's in a simulation.
* In ''Ship in a Bottle'', a holodeck malfunction that causes handedness to flip serves as the crucial clue to figuring out what Moriarty is up to.
* In ''HarpersIsland'', Abby realizes the identity of the murderer, [[spoiler:Henry Dunn]], when [[spoiler:he claims not to have seen his friend Sully, when Abby knows that [[TheCavalry the coast guard]] spoke to both men at the same time]]. It's a bit late by then.
* ''LawAndOrder'' occasionally has the lawyers look at the defense counsel's witness list, and realize that expected witnesses are missing. Upon talking to said not-witnesses, they usually find something that would've sunk the defense's case. And it does. The detectives occasionally pick up on stuff like this too.
** An episode of SVU had the detectives realizing that their murder victim was in fact [[DeadPersonImpersonation alive and masquerading as her own twin]]. Her husband never suspected, but when questioned by the detectives, realizes that three years prior (when the victim presumably disappeared) that his "wife" suddenly stopped going to church whereas she had previously gone every day and claimed to have misplaced her crucifix, which had in fact been buried with the victim.
* Played straight and then subverted in an episode of ''KennyVsSpenny''. In the "Who Can Be Obese Longer?" competition Kenny uses extensive prosthetic makeup to make himself look like a fat person. What this lets him do is remove the extra weight they're forced to wear without Spenny noticing. After a while Kenny comes home and Spenny notices something: Kenny lost the top of his index finger as a child, yet Kenny in the fat suit has it. Spenny concludes that Kenny is faking and uses some spy equipment to see that Kenny is in his room playing video games without his weights on. Spenny confronts "Kenny" and takes off his weight. Then the subversion kicks in: [[spoiler:Kenny had a prosthetic finger applied when he was getting the fat makeup put on because he knew Spenny would notice it. Kenny revealed himself to be the real deal and to still be wearing the weights (he put them back on) and won the competition. The "Kenny" in his room was someone made up to look like him.]]
* Subverted in an episode of ''Series/StargateSG1'' where the team finds themselves in a cold-war era military base and are mistaken for spies. When an interrogator asks in Russian if they're spies, Daniel, the (American) linguist, replies "Nyet"...
* {{Soap Opera}}s like to use this with evil twins, etc. Most of the time, the surrounding characters are completely fooled until the real person is able to escape and confront them, but a handful of story lines have averted this. AllMyChildren's Janet pretended to be her sister Natalie. Not only did Natalie's fiance Trevor already suspect this after sleeping with Janet, Janet said [[SomethingOnlyTheyWouldSay something that Trevor had heard Janet say previously]]. ''Series/DaysOfOurLives'' Hope was replaced by the evil princess Gina, but her son and husband suspected this immediately, as Gina's demeanor was icy and cruel, in stark contrast to Hope's. And on GeneralHospital, Mac was replaced by a double who imitated him a little ''too'' perfectly. Plus, the imprisoned Mac fed the double false information about his love life, resulting in him ignoring a fiancee he had previously been very loving towards while making advances (which he thought were welcome, but came off as sleazy, thanks to the other woman's lack of interest in him) to another woman. It was then that his friends wised up to what was going on.
** Also on ''AllMyChildren'', Janet hired an actor to play Will, Dixie's dead brother, with the intent of terrorizing Dixie. Dixie feared she was going off the deep end until another character mentioned a run-in with "Will". Suspicious, Dixie went to the cemetery (where "Will" had supposedly been haunting her) and when "Will" appeared, began interrogating him about a nursery rhyme from their childhood. Of course, he couldn't answer. To top it off, when he tried to run, he was confronted by her husband, thus proving himself to be made of flesh and blood rather than a ghost.
* Averted on ''{{Fringe}}'' when Olivia and her alternate universe versions [[spoiler:exchange places. Their friends and colleagues begin to suspect something is amiss but don't figure it out on their own before other factors reveal the ruse.]]
** Olivia [[spoiler:has been given most of the memories of her alternate and made to think that she was the alternate. Her slips ups in the facade actually save her live at one point as following correct procedure would have gotten her killed. Others attribute her strange behavior to the injuries she supposedly received in an explosion.]]
** [[spoiler:The alternate Olivia actually has to fake things based only on files and a very thorough briefing. She masks her slip ups as 'trying new things'. Peter starts getting suspicious but is distracted by them becoming a couple. Liv is a wee bit miffed when she gets back.]]
* ''Series/DoctorWho''
** Attempted in "Amy's Choice". The TARDIS team are being flipped between two realities, and they need to work out which one is too fantastic to be real--but Rory points out that this is difficult but when you're in a time machine that's bigger on the inside with a bow tie-wearing alien. [[spoiler:Not to mention the fact that ''both'' realities were fake - only the Doctor realised the Dream Lord had already given that fact away.]]
** In "Day of the Doctor", the Doctor forces peace talks (to prevent detonation of a nuclear bomb) between three U.N.I.T. members and three shapeshifting aliens copying them by making everyone forget which is the original and which is the duplicate. One of the humans is asthmatic. Her duplicate inherited the asthma, but not the inhaler, causing both to realise who is who. They keep quiet to allow the peace talks to continue, revealing that the Doctor's plan had worked.
* On ''{{Warehouse 13}}'', in Season 1's "Duped", Pete says that Alice's impersonation of Myka failed when she [[FakeoutMakeout kissed him]]. The real Myka would never have done so. However, that was a lie to ensure that she didn't find out that the rest of the Warehouse staff had figured it out. The real instance of this trope was when Leena used the Farnsworth to call the fake Myka and ask about her pet ferret. Alice gives herself away when she says that she hadn't named it, which Artie and Leena both knew to be false.
* In an episode of ''Series/HogansHeroes'', the [=POWs=] suspected that a German spy has been placed among them. The suspect, who passed the usual questions about American society and such, told them that he didn't speak any German. They staged a fake fire while the man is sleeping, and as the suspect is waking to the confusion, Kinch, who spoke fluent German, told him to go out of the window, which was located across the room. When the suspect immediately went there and started to try to get out, the men knew he was a spy.
** Another episode had a spy inserted by Major Hochstetter that averted most of the blunders that other infiltrators in the series usually commit: he knew the correct codephrase, didn't fall for any bluffs, was up to date on minute details of Americana, had an impeccable cover story and had trained himself not to respond to German. The ruse is only exposed when, during a visit by Hochstetter to Stalag 13, Hogan sees one of the Major's SS goons snap to attention as the spy passes by, as they are trained to do for SS officers but would never have done for a POW.
* ''Series/TheWalkingDead'': Daryl figures out that [[spoiler: it isn't really his brother Merle]] talking to him because [[spoiler: he still has [[LifeOrLimbDecision both his hands]].]]
* A season one episode of {{Sanctuary}} involved an abnormal that uses mind manipulation to impersonate people before killing them. It impersonates Magnus and almost kills Will until it asked for coffee, which Magnus hates, instead of tea.
* The ''Series/Car54WhereAreYou''' episode "Here we go again", the plot is ''started off'' with this. An old criminal discovers from one of the main characters that the only thing they had done wrong on a heist they pulled many years ago and served jail time for was that they had disguised as police wearing ''winter'' uniforms instead of summer uniforms. This set off a funny episode as the criminals pull together their now-aged gang to pull off the heist correctly this time. [[spoiler: They pull it off, but are all too weak to pick up the gold bars. They then escape the bank wearing the correct summer police uniforms... from the 1920's, not the PresentDay (at the time the episode aired) 1960's uniforms.]]
[[/folder]]

[[folder:Tabletop Games]]
* The [[VoluntaryShapeshifting Demoreans]] of ''TabletopGame/{{Timemaster}}'' are prone to being caught this way. Their obsession with "Perfection" means that when they capture someone and take his place, they ''don't'' include any scars or "deformations" (such as a missing fingertip) that the original may have had.
[[/folder]]

[[folder:Video Games]]
* ''VideoGame/MetalGearSolid'': One of the signs that the DARPA chief was Decoy Octopus were that he referred to the terrorist act as the "revolution". No one picked up on it. Despite knowing there was a master of disguise among the enemy, it didn't occur to anyone on Snake's side that he might be impersonating someone.
** ''VideoGame/MetalGearSolid2SonsOfLiberty'' featured Iroquois Pliskin, a Navy SEAL. There are a few clues that he's not quite who he says he is:
---> He claims to have arrived on the Big Shell via fast rope onto Strut E with the rest of the [=SEALs=] [[spoiler:but as he says this, one of the sea lice from the undersea dock in Strut A - the way Raiden got in - crawls away from his leg.]]\\
He gives inspiring mottos in the conversation after he and Raiden locate Stillman [[spoiler:but, rather than giving the [=SEALs=]' motto, he utters those of the British Special Air Service and US Marine Corps.]]
** There's another one in the fourth game. [[spoiler:The ending of Act 3: "Big Boss's Corpse" is missing the left eye. Big Boss lost his RIGHT eye. ''Solidus'' lost his left eye.]]
* In ''Warnings at Waverly Academy'', NancyDrew has the chance to notice that [[spoiler: one of the students is actually twins, who take turn attending classes because only one of them was awarded a scholarship. The Spottable Thread is that one twin's bangs are always falling into her eyes.]]
* In ''EatLeadTheReturnOfMattHazard'', at one point an impostor QA shows up, and they each accuse the other of being the impostor, but Matt already knows the red one is bad. How? Well, aside from GoodColorsEvilColors, she actually flirted with Matt, and gave nothing but bad advice. It helps that Matt is generally GenreSavvy.
* ''VisualNovel/AceAttorneyInvestigations 2'' has a prisoner impersonating a security guard, only to get uncovered when his pet dog goes to him.
* ProfessorLayton and company catch the villainous Don Paolo multiple times throughout the series by way of this. His mistakes range from [[spoiler: [[ProfessorLaytonAndTheCuriousVillage "Inspector Chelmey"]] flipping out over sweets; when the actual inspector is quite fond of his wife Amile's [[BluffTheImpostor (who is not "Amy")]] sweet potato fritters,]] to [[spoiler: [[ProfessorLaytonAndTheDiabolicalBox "Flora"]] descibing a picture 'she' couldn't have seen,]] to [[spoiler: [[ProfessorLaytonAndTheUnwoundFuture "Future Dean Delmona"]] having gray hair in the future despite secretly telling Layton that he wears a hairpiece.]]
* In ''TheForceUnleashed 2'', [[spoiler: Starkiller realizes that a droid is impersonating Juno Eclipse because the real Juno had been shot in the shoulder earlier.]]
* Set up and subsequently subverted in ''VideoGame/AssassinsCreedBrotherhood'' [[spoiler:when Ezio has Bartolomeo pretend to surrender to his own men who are disguised as French troops]]. A guard at the main gate asks their business ''in French'' and Ezio is able to hold a conversation in it. The guard then asks where in France he's from, Ezio is able to provide an acceptable answer. Apparently, [[HandsomeLech his womanizing days]] [[LampshadeHanging paid major dividends almost a quarter-century later]].
** Every game of (Advanced) Wanted requires this. The newbies who run on rooftops openly aren't even trying, but to spot more skilled players demands that you know what [=NPCs=] will and will not do. Then again that might not save you...
* Metal Slug 3 pulls this off nicely when you first meet Morden, [[spoiler: It actually isn't him; his eye patch is on the wrong eye.]]
* A spoileriffic example from ''VideoGame/ChronoCross'': Kid doesn't realize that [[spoiler:Serge and Lynx have swapped bodies]] until [[spoiler: the suddenly talkative Serge]] mentions the name "Lucca", which up to this point hadn't been said by anybody. She can't really do anything about it, as before she realizes what really happened [[spoiler:she gets stabbed for her troubles.]] Also, seeing [[spoiler:[[HeroicMime Serge]] [[SuddenlyVoiced start to talk]]]] is usually a Spotting The Thread moment for the ''player.''
* During the fight with [[spoiler: Vlitra's Core]] in ''AsurasWrath'', the cutscenes before and after the fight show [[spoiler: What look like Golden threads woven by The Golden Spider to hold Vlitra inside the planet itself and holds up it's stage]].
* Learning behaviors of enemy spies in disguise is a crucial skill required to successfully combat them in ''VideoGame/TeamFortress2''. The fact that Spies can only mimic the appearance of a different class means that various aberrant behaviors by "teammates" may mean that they're really not. Things like not shooting their guns, scouts not running fast enough, seeming want to always be behind teammates, colliding with a "teammate" (you can normally walk right through them)... [[http://wiki.teamfortress.com The Team Fortress Wiki]] has a [[http://wiki.teamfortress.com/wiki/Spy_checking list of threads]] to be aware of and look out for (as well as other spy counter-strategies) to avoid a knife in your or your teammates' backs.
** Inverted in the in-game comic "[[http://www.teamfortress.com/shadowboxers/ Shadow Boxers]]", where Miss Pauling is able to tell the Soldier ''isn't'' delusional about infiltrating the robot base when he mentions that Gray Mann was raised by eagles, a fact known only to her and the Administrator.
* In ''VideoGame/GodOfWar: Ascension'', Kratos sees through Tisiphone's illusion because she was wearing the ring of his wife Lysandra.
* The multiplayer espionage game ''Spy Party'' is all about this. The person playing the Spy has to act as closely to the [[{{NPC}} guests]] as possible while completing their objectives, lest the person playing the Sniper realize they are the spy and shoot them. If you're playing against a really good Sniper, even something as small as looking at a statue and putting it down before the animation is about to loop can blow your cover.
[[/folder]]

[[folder:Web Comics]]
* Shape shifting fae Nutmeg from ''[[DanAndMabsFurryAdventures DMFA]]'' 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).
* In [[http://www.girlgeniusonline.com/comic.php?date=20061206 this strip]] of ''Webcomic/GirlGenius'', despite being in pain from [[spoiler: 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... [[spoiler: and he finally recognizes she's (possessed by) Lucrezia from hearing her gloat.]]
* At one point in ''Webcomic/TheOrderOfTheStick'' Elan is replaced by his EvilTwin 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, [[CloudCuckooLander 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.
* In Webcomic/CuantaVida, Jeremy's [[spoiler: green eyes]] and relaxed attitude tip off Jordi that he's dealing with [[spoiler: Miguel, the RED spy]].
[[/folder]]

[[folder:Web Original]]
* At times, ThisVeryWiki. 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 [[spoiler:completely unnecessary]] spoiler space or adding [[spoiler:innocuous]] information under spoilers in strategic places, and making spoilers [[spoiler:longer.....yes, longer]] than necessary so their length doesn't give anything away.
* Parodied in Website/SFDebris's reviews of ''Series/StarTrekVoyager'', in which it turns out that all the hallucinations and fantasies share one common flaw: [[DullSurprise Chakotay]] is ''too lifelike''.
[[/folder]]

[[folder:Western Animation]]
* ''WesternAnimation/TheGrimAdventuresOfBillyAndMandy'': Nergal Jr. tries to impersonate Mandy, but Billy can tell it's off because [[PerpetualFrowner she was smiling]]. Weird use of this trope, as Nergal Jr. keeps his distinct glasses even when [[PaperThinDisguise disguised as others]].
* ''{{Spider-Man The Animated Series}}'': Chameleon duplicates Nick Fury's appearance from his newspaper obituary, but the paper accidentally flipped the photo, putting his eyepatch on the wrong side. Spider-Man noticed the difference immediately.
** Later in the same episode, Chameleon poses as Peter Parker to get into position to kill his target. Spiderman easily spots him and takes him out. As Chameleon is being taken away, he demands to know what was wrong with his disguise, at which point [[BlatantLies Spiderman just makes something up]].
* In the introductory episode of the Secret Society in ''JusticeLeague'', Clayface attempts to lure Batman into a trap by impersonating TheFlash and calling for "help" against a confederate super-villain. Batman arrives on scene to find that "The Flash" has things well in hand:
-->'''"The Flash":''' Looks like I didn't need your help after all, yo. But you can help me tie him up--\\
'''Batman:''' ''(throws a batarang at Flash)'' Catch.\\
''(batarang hits and electrifies Flash, reverting him to Clayface)''\\
'''Batman:''' The ''real'' Flash would have been too fast for that.\\
'''Clayface:''' ...What gave me away?\\
'''Batman:''' You overplayed your part, "[[IronicEcho yo]]".
** From the same show, Lex Luthor needs barely a glance at the Justice Lords (currently masquerading as the Justice League) to know that, "It's not them." Of course, this might have something to do with all of them wearing completely different costumes.
*** Superman had just lobotomized someone.
** In ''WesternAnimation/BatmanBeyond'', something similar happens when the Old Bruce Wayne realizes that "his" voice in his head is actually somebody else, since this voice calls him "Bruce" while he calls himself "Batman".
** In an earlier crossover episode of ''WesternAnimation/SupermanTheAnimatedSeries'', similar to the Billy and Mandy example above, Robin figures out that something is wrong with Bruce Wayne when he smiles on a recorded message.
* ''WesternAnimation/BeastWars'': Megatron makes a clone of Dinobot that can't transform. Had Dinobot not [[ExtremeOmnivore taken care of the problem]], this trope would've resulted.
* ''FostersHomeForImaginaryFriends'': In the episode "Bloo's Brothers", Mac takes Bloo to Show-And-Tell, where he's such a hit that all the other kids at school decide then and there to imagine up their own Bloo knock-offs. Unfortunately, despite all their creative spins on the concept (the Bloo clones tend to look ''nothing'' like Bloo, and most are downright ''bizarre''), they're all {{Jerkass}}es at heart just like the original and end up getting dropped off at Foster's, where Bloo wastes no time organizing them into [[strike: an army to take over the world]] a 100-part choir. However, when Mac shows up with two tickets to the [[FictionalCounterpart Ice Charades]], the other Bloos scramble to impersonate the real one in the hopes of going to the show. In the end, Bloo is pitted against a nigh-perfect clone as Mac grills them both to try and determine which is the real one: the fake gives himself away by launching into a {{Glurge}}-filled friendship speech.
* In ''CodeLyoko'' episode "Franz Hopper", a Polymorphic Clone impersonates Franz Hopper to lure the heroes into a trap. Jeremie realizes he must be an impostor after the fake Franz mentions the destruction of his video diary (which occurred in a previous episode) -- something that the real Franz Hopper couldn't have known about, but that XANA certainly did.
* [[WesternAnimation/FamilyGuy "Wait a minute; your ass just sneezed! And horses can't talk! No, no nothing about this adds up at all."]]
* In ''WesternAnimation/YoungJustice'', Aqualad and Garth steal the uniforms of two of Black Manta's henchmen and try DressingAsTheEnemy to get close. Black Manta sees through it pretty much immediately when he notices that Aqualad tucked his water-bearers into his belt.
* One episode of ''WesternAnimation/RegularShow'', "Temp Check", features this. Doug, a shapeshifting mole attempting to take over Rigby's identity, is for the most part successful in convincing Mordecai and his coworkers that he's the real Rigby, but Mordecai sees through the facade for two reasons: the real Rigby whining about being "replaced", and the fact that Doug hugged Mordecai just before that (an act established earlier on in the episode that Rigby himself would never do).
* Agent Jay in WesternAnimation/MenInBlack once had to find a disguised alien ship. A quick look at the skies over New York, and he singles out a blimp. Elle asks why a blimp over Yankee Stadium was odd - Jay notes there's no game that night.
* In ''CodenameKidsNextDoor'', episode U.T.O.P.I.A., Numbuh One crash lands on an island that's a complete kid paradise, which causes him to be suspicious from the start. He finds his team swimming in the island's swimming pool and they all convince him to drop his suspicions and just have fun. Numbuh One was about to, until he remembers, as established in earlier episodes, ''[[SuperDrowningSkills Numbuh Four can't swim.]]'' At that point, it's revealed that he's been trapped in a LotusEaterMachine.
[[/folder]]

[[folder:Real Life]]
* There is a story of a British spy in occupied France during WWII who was exposed after ordering "Cafe noir". A local would have known that, with milk being scarce, the "noir" was redundant as that is all there was on offer.
* 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).
** German spies were poorly trained indeed but they knew that and successfully recruited some subjects of the Crown. Such stories were then disseminated by MI5 to hide the fact that Bletchley Park crew successfully broke the military Enigma code.
* 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 outted 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 (possibly) 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..."
** Worse yet, I've heard it the other way around - an American spy being told by Nazis, after days of interrogation, "Well, seems you're not a spy after all, you can go (in German)" to which they replied, "Thank you!"
*** Likewise, according to QI, female spies can be found out because on giving birth they will scream out in their native language; apparently Himmler found this out when a "German" radio operator screamed in Russian while giving birth.
**** This is a plot point in ''Series/SeventeenMomentsOfSpring'' - and the radio operator is one of the major good characters.
* 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!
** No doubt the spy, like any other German speaker, had frequently needed to correct someone who'd misheard "zwei" (2) as "drei" (3) (or vice versa).
* And another one about German infiltrators during Battle of the Bulge:
-->'''Spy (on American gas station)''': [[BritishAccents Petrol]], [[QuintessentialBritishGentleman please]]!
* This one is real: whenever German spies infiltrated the Soviet army, they usually had perfect and impeccable legends, accents and documents. But the staples on their documents were shiny stainless steel -- the USSR was still stuck with iron staples, which rusted.
** Similarly, the Germans-dressed-as-Americans during the Battle of the Bulge were given away due to their forger's correcting an intentional error in the standard GI identity cards.
** Also, there's a story about the exchanging passwords, where the guard would call, "Oh say can you see," and expect the answer, "by the dawn's early light?" Then the guard would call, "And where is that band," and if the answer was "who so vauntingly swore,", they'd know it was a spy. Because while any red blooded American could be counted on to know the first verse of the Star Spangled Banner, only a spy who was trying too hard would have memorized the ''third''. Asimov used this for a short mystery in ''The Union Club Mysteries''.
*** Guess YouWereTryingTooHard
*** The sheer irony is this probably got some people wrongfully suspected, either from being actual trying-too-hard patriots or supply-chain contientious objectors.
* 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 ([[OldMoney 240 pennies to a pound, quarters of pennies, etc.]]), and the 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.
* 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.
* [[http://en.wikipedia.org/wiki/Shibboleth 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 still useless against traitors):
** The Dutch used the name of the port town Scheveningen to unmask Germans.
** In 1302, during the [[http://en.wikipedia.org/wiki/Bruges_Matins_%28history%29 Bruges Matins]] the Flemish used the phrase "Schild en Vriend" ("Shield and friend") to identify and murder [[CheeseEatingSurrenderMonkeys French sympathisers]].
** 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 nonsensical [[note]]''lentil wheel mills mill''[[/note]] 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: got new clothes, 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.
* In 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, saccharine 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 dealing than the civilians.
* More hapless German and Russian spies in and after WWII ran into trouble when, if they made it to England, they ordered [[SpotOfTea tea without milk whilst posing as working-class brits.]] That would be seen as a little odd today; in the 40’s and 50’s you may as well have turned up with a sandwich board saying “I’m a spy”.
* 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 PlayedForLaughs (in tandem with what appears to be either PoliticalCorrectnessGoneMad or just {{Eagleland}}) in this ColdWar-era joke:
--> An elite American spy, after extensive and gruelling 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]]an affectionate way a Russian may refer to an old woman[[/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 [[VodkaDrunkenski 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 [[PaperThinDisguise we have no Negroes]]. [[note]]You may find the word offensive, but simple-minded Granny probably wouldn't. Consider it part of the joke.[[/note]]
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