Spotting the Thread



""A web of lies can unravel with the lightest touch of the truth!""

- Othar Tryggvassen (Gentleman Adventurer!), Girl Genius

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), or accidentally revealed that they're left-handed.

This can more easily occur during a round of Bluff the Impostor, 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 I Never Said It Was Poison, a more incriminating version. See also Conviction by Counterfactual Clue, when this gets even more unrealistic. Often overlaps with Imposter Forgot One Detail.

When applied to a dream test, it's A Glitch in the Matrix.

Advertising
"Talking dog dressed as ghost: How did you know it was me? Human: Grandpa didn't have a tail."
 * Played for Laughs in a Bush's Baked Beans Ad:

Anime and 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. The Reveal is that it was a Yoma impersonating a Claymore to take him hostage in order to draw Clare out and kill her.
 * Twice in Fullmetal Alchemist. The first time is where Maes Hughes catches out the shape-shifter Envy because he forgot a 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, but then admitting that she lied when he panicked.
 * Played with in the case of the latter in that she fakes Spotting the Thread in order to expose the actual thread.
 * Mahou Sensei Negima: The Takamichi illusion during the Bad Future Rescue Arc.
 * Also done humorously, when Negi realizes that the Shizuna-sensei he's speaking to is an impostor because of a Marshmallow Hell. Turns out that the impostor (Asakura) didn't quite measure up to Shizuna's bust size.
 * 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, Kohonamaru is spotted trying to trail Naruto, as he is holding his fence-colored camouflage in the wrong direction.
 * In the Lupin III Versus Detective Conan movie, Zenigata identifies the disguised Lupin when he calls him Totsan, as he always does (in the Japanese version).
 * In the Alabasta arc in One Piece, 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.
 * In Monster, Tenma once picks up on two "friendly" helpers being accomplices of Johan when one of them refers to him as "doctor"—something he never told them about.
 * Although he'd noticed a few clues beforehand; that slip was just the confirmation he needed (as well as his not being stuck in a car with them at the time).
 * Axis Powers Hetalia: 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 Sailor Moon R one of the Monsters of the Week disguise themselves as Rini's mom and calls to her using the name Small Lady. Rini 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, Rini say she's lying because her mother never calls her Small Lady, her royal title.
 * In the "Allison" arc of Allison & Lillia, Wil spots an intelligence agent when said agent mentions something his public persona could not have possibly known, but Wil did know.

Comic Books
"Deadpool: Which I signed up for as Wade Winston Wilson!"
 * In the Batman crossover 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.
 * The 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 awesome kicking of his ass as a result.
 * Deadpool got a memory/flashback version in Cable and Deadpool-when T-Ray showed Deadpool the moment when Deadpool "tried to kill him and then stole his identity" in an eariler 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 becuase his cancer made him leave the army-


 * This gets retconned in and out Depending on the Writer, though fans typically take this as proof that Deadpool is Wade and T-Ray is not.
 * In one comic, Tintin disguised himself, but failed to disguise his dog.
 * Lucky Luke: 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.

Films -- Live-Action
"Steve: Still don't trust me? Stella: I trust everyone. It's the devil inside them I don't trust."
 * Harry Potter 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.
 * In X2, 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 1974 film Godzilla VS Mechagodzilla, 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 Inglourious Basterds, An honest mistake usually, but this is Quentin Tarantino, 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 The Great Escape, 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 Inception there's an example of literally spotting the thread, when
 * In Attack of the Killer Tomatoes, "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 The Italian Job 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.

"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 the documentary Catfish, Nev begins to realize because he finds out she took other people's songs and said they were hers.
 * The dramatic thrust of Sucker Punch is almost entirely dependent on whether or not the audience can do this. There's a really big clue, though, just before The Reveal that a twist is coming because a character's wearing a dress she shouldn't have.
 * In Star Trek IV: The Voyage Home, 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.

"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."
 * 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:

Literature
"Eisenhorn: You could talk like and use Glossia, but didn't know what  knew. We fought Sadia on Lethe Eleven, not Eechan."
 * In Animorphs, when Jake is infested with a Yeerk (mind-controlling slug), the Genre Savvy 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). Yeerk blows the disguise completely when being touched by Ax, which revolts him beyond limits, forces him to blurt out a glaring rant at Ax.
 * 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 them, just to make absolutely sure. 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.
 * 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.
 * 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
 * Also in Hallows, the Death Eaters realize which one of the several Harrys are the real one when he uses his "signature" spell, Expelliarmus.
 * 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 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 Did Not Do the 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.
 * At this time, presumably all girls learn to sew, but barely any boys, I guess.
 * 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 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.
 * In Faded Steel Heat, Garrett is impersonated by, 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 The Book of the New Sun, Severian spots a literal thread:
 * 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? 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.
 * Eisenhorn:


 * In the Georgette Heyer novel False Colours, Kit is caught in a Twin Switch when he opens a snuffbox the wrong way.
 * One science fiction novel had the protagonist 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.

Live-Action TV

 * 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.
 * Red Dwarf: TWO examples in the season 6 opener "Psirens"; the first being when a psiren masquerading as Kryten refers to Lister as 'Dave', and the second when a psiren masquerading as Lister gives himself away by playing the guitar too well.
 * 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.
 * 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 Latex Perfection mask.
 * Star Trek: Deep Space Nine 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!".
 * Star Trek: The Next Generation 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 pictures of his dead wife were of a (non-existant) girl he met on the holodeck, Geordi takes over 30 hours to run a level one diagnostic ("You're incapable of this level of incompetence!"), Data cannot do trillions of calculations per second, and can use contractions.
 * The use of contractions was the biggest clue, because Data not using them is involuntary.
 * In Harper's Island, Abby realizes the identity of the murderer,, when . It's a bit late by then.
 * Law and Order 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 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 Kenny vs. Spenny. 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:
 * Subverted in an episode of Stargate SG-1 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 Operas 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. All My Children's Janet pretended to be her sister Natalie. Not only did Natalie's fiance Trevor already suspect this after sleeping with Janet, Janet said something that Trevor had heard Janet say previously. Days of Our Lives 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 General Hospital, 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 All My Children, 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. Off 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
 * Olivia
 * Attempted in Doctor Who. The TARDIS team are being flipped between two realities, and they need to work out which one is too fantastic to be real- bur 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.
 * On Warehouse 13, Pete spots Alice's impersonation of Myka when she kisses him. The real Myka would never have done so.
 * On Warehouse 13, Pete spots Alice's impersonation of Myka when she kisses him. The real Myka would never have done so.

Video Games

 * Metal Gear Solid: 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.
 * To be fair, five minutes after meeting Octopus as DARPA chief, Octopus suffers a fatal heart attack and dies. In front of Snake, complete with scream of pain and mouth foaming. Even knowing there is a master of disguise among the enemies, I don't think anyone expected him to be that committed to the role.
 * Metal Gear Solid 2 featured Pliskin, who said "Semper Fi", which gave him away as not being a Navy SEAL.
 * There's another one in the fourth game.
 * In Warnings at Waverly Academy, Nancy Drew has the chance to notice that
 * In Eat Lead: The Return of Matt Hazard, 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 the fact she's Color-Coded for Your Convenience, she actually flirted with Matt, and gave nothing but bad advice. It helps that Matt is generally Genre Savvy.
 * The Ace Attorney games have a lot of these, such as in Investigations.
 * Professor Layton and company catch the villainous Don Paolo multiple times throughout the series by way of this. His mistakes range from  to   to
 * In The Force Unleashed 2,
 * Set up and subsequently subverted in  Assassin's Creed: Brotherhood  . 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, his womanizing days 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,
 * A spoileriffic example from Chrono Cross: Kid doesn't realize that until   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  Also, seeing  is usually a Spotting the Thread moment for the player.
 * During the fight with  in Asura's Wrath, the cutscenes before and after the fight show.

Web Comics

 * Shape shifting fae Nutmeg from 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 this strip of Girl Genius, despite being in pain from, 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...
 * at one point in The Order of the Stick 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.
 * In Cuanta Vida, Jeremy's  and relaxed attitude tip off Jordi that he's dealing with.

Web Original

 * 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.

Western Animation
""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, "yo"."
 * The Grim Adventures of Billy and Mandy: Nergal Jr. tries to impersonate Mandy, but Billy can tell it's off because she was smiling. Weird use of this trope, as Nergal Jr. keeps his distinct glasses even when 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.
 * In the introductory episode of the Secret Society in Justice League, Clayface attempts to lure Batman into a trap by impersonating The Flash and calling for "help" against a confederate super-villain. Batman arrives on scene to find that "The Flash" has things well in hand:


 * 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.
 * Not to mention that Superman had just lobotomized someone.
 * In Batman Beyond, 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 Superman: The Animated Series, 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.
 * Beast Wars: Megatron makes a clone of Dinobot that can't transform. Had Dinobot not taken care of the problem, this trope would've resulted.
 * Foster's Home for Imaginary Friends: 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 Jerkasses at heart just like the original and end up getting dropped off at Foster's, where Bloo wastes no time organizing them into an army to take over the world a 100-part choir. However, when Mac shows up with two tickets to the 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 Code Lyoko 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 mention 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.
 * "Wait a minute; your ass just sneezed! And horses can't talk! No, no nothing about this adds up at all."
 * In Young Justice, Aqualad and Garth steal the uniforms of two of Black Manta's henchmen and try Dressing as the Enemy 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 Regular Show, "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).
 * In the climactic battle in Megamind, Titan realizes that Metroman is actually.

Real Life
"spy: Two martinies, please. bartender: Dry? (homophonic to German "drei" - "three") spy: Nein, zwei!"
 * 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).
 * There were stories of British spies nearly getting caught for putting spoonfuls of soup straight into their mouthes (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..." Ouch.
 * 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 Seventeen Moments of Spring - 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 (on American gas station): Petrol, please!"
 * 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:


 * 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 that would be a case of You Were Trying Too Hard
 * 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 (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.
 * 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.
 * 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 Bruges Matins the Flemish used the phrase "Schild en Vriend" ("Shield and friend") to identify and murder French sympathisers.
 * 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, MI 6 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 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”