Out of Character Alert

The inversion of Something Only They Would Say: When a character is pretending to be someone else, they may unwittingly reveal this by saying something that would be out-of-character for who they're impersonating. Variations include not responding to a well-known Berserk Button, doing things they're normally afraid of (or have a similar excuse for never doing), or insisting to be called by a nickname they hate.

Often invoked in kidnapping and I Have Your Wife scenarios, to let the heroes know that something is amiss. If the Big Bad is demanding a ransom, this is to alert them to the fact that it's a trap; if he wants the kidnapped to "ensure" The Hero that the kidnapped is "in fact" okay, this is to secretly convey that they're not.

Actual military personnel sometimes use hand signals when being taped to communicate in another way with their 'home base'. There are a few documented cases of soldiers giving hand signals (and one case of them just flipping the bird to the camera) to alert the people receiving it that no, they weren't being treated very politely at all. And some have done it just for fun, giving the sign for coercion when forced to shake a politician's hand, for instance...

Steganography is the art of writing hidden messages that only the intended recipient would recognize.

Compare with Not Himself, contrast with Something Only They Would Say (in which a character is identified by a characteristic) and Bluff the Impostor.

Advertising

 * In an early '70s TV commercial for the short-lived Cap'n Crunch variant Vanilly Crunch, the good captain's ship is approached by Wilma the Whale, the mascot for that cereal. After the captain does a description of the cereal, he shoots a cannon at the whale! Turns out that pirate Jean La Foote (the "bad guy" in these ads) had disguised his now-sinking vessel as Wilma. Cap'n Crunch knew it because he described Vanilly Crunch and the whale didn't smile.

Anime and Manga

 * Used to Hawkeye's advantage in Fullmetal Alchemist to trick Envy, who was disguised as Roy, as part of her Batman Gambit. She aims a gun at him, saying that Roy never calls her "Lieutenant" in private. Cue Envy immediately breaking the form, cursing his luck and shouting "I knew there was something going on with you two!'' Her reply can effectively be summed up as, "Not really. Thanks for believing me, Envy." Cue epic beatdown.
 * This trope is a key plot point in the Weiss Kreuz OAV Verbrechen ~ Strafe. During a mission briefing, when asked if she's well, Weiss's handler Manx replies that she's having trouble with her allergy to black lilies. Much later, when Weiss finish springing their trap on the villain who was holding Manx hostage, they reveal that "black lily" is a codeword indicating "lies," and that Manx's comment had alerted them to the fact that the orders given in that mission briefing were fraudulent.
 * In the Chuunin exams arc of Naruto, Sasuke proposes using a password in case of enemy ninjas using doppelgangers to imitate one of them (again), and makes up a long poem as the passphrase. A few scenes later, Naruto excuses himself to pee, and when he returns he is quizzed for the passphrase, and successfully gives it. Of course, Sasuke knows that the real Naruto would not have remembered such a long passphrase, and that he was being spied on when he made it.
 * Also during the Sasuke Retrieval arc. After defeating Jirobou of the Sound Four, Shikamaru disguises as him and catches up with the other three. When he does, Tayuya cusses him out and asks what took him so long. Shikamaru, still disguised, quietly apologizes. The other three immediately realize he's not Jirobou, as Jirobou always chastised Tayuya for using unladylike language.
 * Used again much later by the Mizukage to.
 * This happens in Fushigi Yuugi when the mirror-Miaka tries to seduce Hotohori to overpower both him and Tamahome. Before she can kiss him, though, Hotohori lifts his sword, saying that Miaka never behaves that way.
 * A variant was used in S-Cry-ed: Kazuma, trapped in Unkei's Lotus Eater Machine Alter, is challenged by that world's Ryuho to a friendly sparring match. They draw, and Ryuho offers him a hand up. This so badly clashes with Kazuma's memory of him that the illusion cracks, and he pulls out his Alter. The illusory Ryuho panics and begs for his life... at which point Kazuma destroys the illusion - the Ryuho he knows would just summon his Alter and counterattack.
 * In Tsubasa Reservoir Chronicle, Sakura is seemingly trapped in a bubble and Syaoran is warned that if he fights back, she'll get hurt. He takes the blows, but when 'Sakura' calls out "Syaoran!", he knows she's a fake because she lost her memory, so she is now polite with him and says his name with the -kun honorific.
 * In the Devil's Trill arc of Yami no Matsuei (Descendants of Darkness), Hisoka figures out that Tsuzuki is possessed by Sargatanas very simply: first, Tsuzuki doesn't take up an offer to go out to eat, something he would never do. Second, Hisoka mentions that he bought their boss salted cuttlefish since he hates sweets, which doesn't get commented on by Tsuzuki. In fact, the boss is infamous for his Sweet Tooth.
 * In Eyeshield 21, at the start of the second half of the Devil Bats' game with the Shinryuji Nagas, Hiruma says their chances of winning are almost 0%, and that the team shouldn't do anything drastic as to harm their future careers.
 * In One Piece, Vivi begins to suspect that there's something wrong with Usopp when he tells her to abandon the injured Carue, referring to him as "that bird." The real Usopp not only knows Carue's name, but is fairly close to Carue. In fact, it was Mr. 2 (Bon Clay) impersonating him in order to trick her.
 * In the Mai-HiME manga, Haruka begins to suspect that her Lotus Eater Machine dream is a fake when her rival Shizuru humbly says that she should never have competed with her. Haruka points out that she finds Shizuru to be quite arrogant, and that she could never be this happy.
 * At one point in Dragon Ball GT, Goku knew immediately that a mind-controlled Vegeta is Not Himself because he was calling him "Goku". The real Vegeta always made a point of referring to Goku by his Saiyan name, Kakarot.
 * In Sailor Moon R one of the Monsters of the Week 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... but she then suddenly stops and asked the monster who they were. When the monster replies that she was her mother, Chibi-Usa replies she's lying because her mother never calls her Small Lady, which is her royal title.
 * In one of The King of Fighters manga, Vice attempts to seduce Kyo while disguised as his girlfriend Yuki. He blows her cover when she's about to kiss him and says that, due to the context they're in, Yuki would actually be upset at him, not hitting on him.

Comic Books

 * In a Spider-Man comic, Eddie "Venom" Brock's ex-wife (who is surrounded by police) addresses him as Edward over the phone. Displaying his typical lack of subtlety, Eddie immediately shouts "WHO'S WITH YOU?!"
 * In one episode of Flash Gordon, Dale distinguishes the real Flash from an imposter when the fake announces that he loves her. In a bit of a twist, Flash (the real one) is visibly troubled by this.
 * Modesty Blaise: When captured by terrorists and forced to call Modesty, Willie addresses her by name instead of calling her "Princess". She immediately packs her bags and comes to his rescue.
 * Modesty & Willie also repeatedly use a pre-arranged distress code throughout the series. If either of them drops the name "Jacqueline" into a communication, that's the cue to hit the panic button.
 * They also use "Bertha" as code for "I need a distraction".
 * Captured by the Mekon and forced to broadcast a message to Earth, Digby claims he's having such a nice time it reminds him of a holiday he once had. His aunt realises he's referring to an occasion when he was wrongfully arrested and that he and Dan Dare are being held against their will.
 * In a Mickey Mouse story a famous opera singer suddenly not only abandons his contract but signs on for a rival - and refuses to see anyone. His former studio, not being idiots, finds this suspicious, but have no proof. When he releases a "greatest hits" collection, however, one word in each of the famous arias (all spoofs of real ones) is wrong - and the wrong words together spell out the location where he is held against his will. This trope truly enters when his ex-manager points out he would never miss a word like that.
 * Horribly averted in Maus. Art Spiegelman's father and mother were hidden in Nazi-occupied Poland but wanted to go to a safer country. Smugglers promised to help them but Vladek Spiegelman wasn't sure if they would betray him or not. A friend went first and wrote a letter back saying everything was fine. Unfortunately they didn't set up any kind of warning system and the friend wrote the letter with a gun to his head. They all ended up in Auschwitz including, eventually, the smugglers (ironically the friends who moved into their vacated hiding place survived the war).
 * In Frank Miller's first issue of Daredevil, Elektra forces a Mook to phone his boss and set up a meeting (so she could capture him and collect his bounty). The mook suggests a time an hour later than previously planned, which was their codeword for trouble. His boss knew he was going to be walking into a trap.
 * In one issue of Doom Patrol, Larry figured out that the putty-like Madame Rouge was impersonating the Chief when "he" called Rita Elasti-Girl -- "the Chief would NEVER call Rita by that freak name!"
 * Barbara Gordon and Ted Kord know each other well. A robot impersonates his voice, yet "Babs, fortunately, is no fool, and knows Ted far too well to fall for that shit."

Fanfiction

 * In a MLP Dresden Files crossover fic The Dresden Fillies, Harry discovers this trope in his own Inner Monologue. This clues him to the fact that

Film
"John: Something's wrong, she's never this nice."
 * In Sneakers, Sidney Poitier's character draws Robert Redford's away from a meeting with the villains in a public meeting space after seeing a newspaper blurb mentioning that the mathematician they had stolen the MacGuffin from has been killed and realizing what their employers really are. He gets Redford to leave with him by holding up the carphone and shouting that it's his mother calling, in what's clearly a pre-arranged disaster code (Redford's character has been living under an assumed name for the last twenty years...).
 * Also, because one of their group has the nickname "Mother"...
 * Terminator 2 does this twice, first when John tries to call his foster parents to warn them about the T-1000:

"The Terminator: [to John] What's the dog's name?
 * At which point the Terminator takes over:

John: Max.

The Terminator: [in John's voice] Hey Janelle, what's wrong with Wolfie? I can hear him barking.

"Janelle": Wolfie's fine, honey, Wolfie's just fine. Where are you?

The Terminator: [hangs up the phone] Your foster parents are dead."

"Mal and Inara have a polite, awkward conversation by videophone, with Inara inviting Mal to visit her. Discussion with the rest of the crew ensues.
 * It happens again when the T-1000 takes Sarah Connor's form and asks for help, something the real Sarah had previously berated her son for doing. The other Sarah instead tells John to get out of the way, something the T-1000 would not be inclined to tell his target.
 * In the Directors Cut, however, John is tipped off when he sees his "mother's" foot melting into the floor.
 * The Firefly movie, Serenity

Zoe: So. Trap?

Mal: Trap.

Kaylee: How do you know Inara don't just wanna see you? People do have feelings. I'm referrin' here to people.

Mal: Y'all were watchin' I take it?

Zoe: Yes.

Mal: You see us fight?

Kaylee: No...

Mal, matter-of-factly: Trap."


 * The DVD release includes an amusing alternate take where Mal instead screams that last "TRAP!" at the top of his lungs.
 * Also, the first hint Simon had that River was being mistreated was that her letters to him were full of spelling errors and referenced family outings that never happened.
 * Subverted in The Bourne Ultimatum, when
 * And it becomes a Double Subversion
 * In National Treasure: Book of Secrets, the trap is somewhat different. Mitch Wilkinson forces Emily Appleton to give her ex-husband a rather unconvincing fake translation of some Olmec hieroglyphs. He doesn't seem to pick up on her gestures, so she promptly texts her son Ben Gates the real translation.
 * In the second Lara Croft Tomb Raider movie, Lara's butler tries to do this, but is rebuked for it. "Why are we even having this conversation?"
 * Happened in the second Miss Congeniality movie.
 * Miss United States drops a big hint to her location by saying "My booty is on the line here." Sandra's character catches on and explains that "She would never refer to her butt as her booty... she would never refer to her butt as her butt! She calls it a po-po!" The kidnappers tie her and the celebrity pageant host into the sinking ship at Treasure Island in Las Vegas to passively kill them.
 * Bullshot (1983). The dastardly villain, Count Otto von Bruno, who speaks in a stereotypical Prussian accent, fakes the voice of Professor Fenton with Applied Phlebotinum. Despite a chronic malfunction which forces von Bruno to speak at a vastly sped-up rate, the Professor's dim-witted daughter doesn't suspect a thing when he says: "I want you the formula to London bring." Only our hero Bullshot Crummond realises instantly that you'd never hear a split infinitive from an Oxford man, and so it must be a trap!
 * In Die Hard With a Vengeance, a bunch of German mercenaries impersonate cops. Although the leader speaks English with a flawless American accent, he slips up on a few word choices, such as calling an elevator a "lift" and saying that it's raining "dogs and cats," instead of the usual "cats and dogs. When McClane recognizes that one of them is wearing a friend's badge, and mentions the lottery to figure out if anyone on the elevator is real. None of the fake cops know last night's numbers, though in the beginning it's established that every NYPD cop plays the lottery with their badge number.
 * Since we don't have a "Something They Would Never Do"-Trope: One of the Dirty Harry films had Harry walk into a café that was being robbed. Everyone inside was forced to act naturally while "The Cop" made his order, but the waitress managed to warn Harry by pouring about five tea-spoons of sugar in his coffee after ten years of "His Usual" being No Sugar, No Milk.
 * A similar scene in the film It Could Happen To You. When a cop walks into a local deli to get coffee, he asks where the owner's wife is. The owner tells him that she's out sick, but as he turns to get supplies we see that his wife is being held at gunpoint by a robber. Although the cop doesn't see this, when he goes outside to join his partner, he tells him that the man is being robbed--having come to the store for years, they know that "that woman could be dead and she'd STILL show up for work".
 * Inverted in Single White Female: Allie has just realized the extent of her roommate Hedy's depravity and psychosis when she realizes that she's murdered her boyfriend. Desperate to get out of the apartment without arousing Hedy's suspicion, Allie hurriedly claims that her upstairs neighbor Graham had called her and invited her to hang out with him. Unfortunately, unknown to Allie, Hedy bludgeoned Graham to death the previous night (but as it turns out he was merely unconscious) and knows Allie's lying.
 * In Inglourious Basterds one of the team's plans gets upset by this when, trying to pass for German soldiers and ordering drinks, they use the American gesture for the number three instead of the German version. The real German soldiers notice this and promptly try to eliminate the imposters.

Literature

 * Animorphs has a straight example when Jake is infested. His friends realize he has been body snatched when he shows sudden, inexplicable revulsion to Ax; the accusations tick the Yeerk off so much that he eventually snaps and calls him "Andalite filth," confirming the suspicions. Of course most of the time the trope is subverted with Yeerks impersonating humans flawlessly.
 * And then he flubbed it again by laughing when he realized that Ax's plan to take his place for three days while the rest of the group starved the Yeerk out of his head would most likely crash and burn. Obviously, a bad Jake impersonation and a suspicious Tom wouldn't be a good thing for the real Jake, no matter how annoyed he was with his friends.
 * That particular Yeerk seems to be a bad actor; he was also Tom's original Yeerk, who quit the basketball team and thus clued Marco in that Tom was Not Himself.
 * Georgina "George" Kirrin in the first The Famous Five novel is ordered to send a note to two of the gang (another is with her) by some smugglers. She signs it "Georgina", which she would never call herself and gets highly offended when called that.
 * In Green Rider, a man hides a crucial message in a love letter. He's killed before he can deliver it, but the protagonist, Karigan, passes on the love letter to the messenger's girlfriend. When she gets the letter, she spots some inconsistent details (he misstates the color of her hair; he mentions a brother when he doesn't have one) and mentions it to Karigan, who realizes that the letter contains a coded message.
 * Agatha Christie's short story "The Adventure of the Sinister Stranger" uses a Homage to the now-obscure Oakwood Brothers stories by Valentine Williams. When Tommy announces he is going to "walk into a trap with my eyes open", Tuppence says that this is exactly what happens in the stories when Desmond Oakwood blunders into something and needs Francis to rescue him. Tommy subsequently signs a letter asking Tuppence to return to the agency "Francis", to signal that he's done precisely that.
 * In The Secret Adversary, Tommy realises that a message sent to him from Tuppence is not actually from her, as it is signed "Twopence".
 * Also used in the novel The Man in the Brown Suit. After Anne is lured into a trap by a note allegedly sent by Harry, she and Harry decide that in all future written communication, they will only refer to each other by pseudonyms. Later, Anne is kidnapped and forced to write Harry a note which will lead him into a trap. When she signs it with her real name instead of the agreed-upon pseudonym, Harry recognizes that the note is a fake and that Anne is in trouble.
 * In the science fiction novel Sewer Gas and Electric, one of the main characters (a parody of Ayn Rand's heroes) is forced to play a twisted computer game against a robot double of himself, with his parents' lives at stake. When his ex-girlfriend charges in with a gun to rescue him, the fake jumped up and acted relieved, whereupon she gunned it down. The real one was so absorbed in the game that he didn't even notice any of this until several turns later, when he realized his opponent hadn't made any moves.
 * One of the Star Wars Expanded Universe books from the eighties - Han Solo's Revenge - inverted the trope. When Han and Chewie were to make a smuggling drop, if during the meeting Han did not try and signal Chewie (who was overseeing the proceedings back in the cockpit of the Falcon) then something had gone wrong with the drop.
 * A Series of Unfortunate Events presents a written variant in the third book. The trio's Grammar Nazi auntie leaves them what looked like a suicide note, but was filled with mistakes, leading them to deduce that it was written under duress and contained some hidden message. Which it did: the letters involved in the misspelled and malformed words spelled out where she was actually hiding.
 * In the Jack Reacher novel "Tripwire", his girl-friend has been captured and been ordered to lure him into a trap. She calls him up and opens the conversation with "Hi, Jack". The point is that the main character is *always* called Reacher, by everyone including his mother when he was very young, and no one ever uses his first name. The coincidence of "Hi, Jack" and "hijack" only makes it more convincing that this is indeed a trap.
 * In a short mystery story involving a group of women at some sort of get-together, one of the members sends a note saying she won't be able to make the meeting. The women reading the letter note discrepancies in the letter, each discrepancy referencing a number (including the absent member's middle initial in her signature being erroneously given as "O," which is taken to mean "0.") There are a total of seven errors in the letter, and when the errors are written out in their numerical form, one of the group theorizes it may be a telephone number. It is -- to the police.
 * In Fearless, there's one arc where Gaia is being forced to humiliate Ed Fargo. He asks her "You're enjoying this aren't you?" and Gaia answers, "Yes, I like torturing you. Almost as much as I like Lox." This is actually more of a coded message, as Gaia hates Lox and is trying to tell Ed she is being forced to do this. However, she would never say she likes Lox, so it works.
 * In the second Lady Grace mystery, Lady Sarah is abducted by a sea captain, and alerts Grace/ anyone who can help with a message passed by a commoner that she sends her love to " Lady Jane, my dearest friend". the two young women hate each other with passion, and so Grace and Masu are off to the rescue in a trice.
 * In Jennifer the Jerk Is Missing, there is an unusual variant of this. To find out if Jennifer-the-Jerk Smith made it to camp or was kidnapped before reaching it, the protagonist, Amy, calls the camp to see if Jennifer Smith arrived. She's told that yes, Jennifer Smith did indeed arrive, and she's a very pleasant and charming girl. Malcolm, the kid who suspected the kidnapping in the first place, immediately recognizes that Jennifer-the-Jerk Smith is neither pleasant nor charming, and therefore the girl that arrived couldn't have been her. (She wasn't)
 * In Wayside School Gets A Little Stranger, the students' voices are stolen by evil substitute teacher Mr. Gorf; when the lunch lady asks how the class is doing from outside the door, Mr. Gorf tries to trick her by using the voices of the students to say that everything is okay. The lunch lady figures out something is wrong, though, when the voice of the meanest student in the class says "Have a nice day!"
 * One old spy story had a member of the French Resistance of World War II giving out the duress code while they were under duress and having their British contact in England tell them they made a mistake, "That's the duress code, you need to remember not to use that." Needless to say, things went downhill from there.
 * In The Bet's On, Lizzie Bingman, the titular character is kidnapped because she witnessed a murder. While she's leaving with the murderer she manages to alert her friend because she mentions her sisters: Lizzie only had brothers.
 * In Mirror Dance, Bel Thorne suspects that the "Admiral Naismith" who just boarded the ship is really Mark and not Miles, and confirms it by referring to Mark as Miles' clone in conversation. The real Miles always refers to Mark as his brother and corrects anyone who says otherwise, but Mark doesn't know this and lets it slide.
 * In the Tom Clancy/Larry Bond novel Red Storm Rising, Air Force weatherman Mike Edwards, stranded on occupied Iceland and radioing NATO everything he sees, is given a Duress Code. Played with in that he nearly says it by mistake. (If captured and made to phone in phony reports, he is supposed to preface the message with "Beagle Calling Doghouse, things are going great.)
 * In The Dresden Files: Blood Rites, after the two of them are captured by the Big Bad, Lord Raith, Murphy calls Harry "Mister Dresden." Raith dismisses it without a second thought, but Harry immediately picks up on it as her playing up being helpless, as it went against both the nature of their friendship and her feminist personality.
 * In a Nancy Drew book, a young woman on the phone with Nancy asks her to "tell Ned I'll see him at the big rally on Monday". There is no rally on Monday--the girl is trying to tell Nancy and Ned that she's in trouble. Unfortunately, not realizing the importance of the message, Nancy doesn't relay it to Ned.
 * In World War Z the Chinese doctor who encountered one of the first victims of the zombie plague had a friend working in the government. This friend was an eternal pessimist; no matter the situation, he'd always assume it was going to get worse. When the doctor tells his friend about the victim over the phone the friend says, "Don't worry. Everything's going to be all right." That's when the doctor knew that things were really bad.
 * John Birmingham's Axis Of Time: A temporally-displaced multinational fleet from 2021 and the US Pacific Fleet of 1941 have just engaged in battle by accident, and are trying to sort out the situation. One of the 1941 sailors volunteers to go over to the future fleet, and arranges a duress signal with his superiors by suggesting "My sainted mother taught me never to swear, so if anything is wrong, I could slip in a fucking profanity, sir."
 * In Ender's Game, Ender gets a message that really was written by his sister Val, but he figures out that something is off about it. He correctly guesses that the military told her to write it and told her that Ender wouldn't receive it unless she wrote what they wanted her to.
 * More to the point, Valentine adds so many personal in-jokes and idiosyncracies that Ender can't help but assume that she was told to write the letter in such a way that it was stupidly obvious that it was her.
 * Lemony Snicket: The Unauthorized Autobiography features a long and poignant letter written by the Duchess R to Lemony Snicket. He immediately lambasts the numerous errors she would never have made.
 * In the LionBoy series, the main character knows his parents are in trouble because their letter to him is written the way an adult talks to a child, while his parents always talk to him like he was older. He and his parents use this in all their communications throughout the series.
 * In one Maximum Ride book, a clone of Max tries to take her place. The kids realize something is up when she offers to cook (as Max is a Lethal Chef and leaves the cooking to Iggy), and when she expresses surprise that Iggy would know his way around considering that he's blind. Of course, Angel can read minds, too, which the clone has no clue of, so she knew right away.

Live Action TV
"Fred: "You don't think he was referring to anything of mine that's fluffy do you? Because that would just be inappropriate...""
 * Played twice in the first six hours of 24: Kim said "I love you" over the phone to her mother Teri after being kidnapped, and Teri said the same to Jack under similar circumstances. (Amusingly, Teri catches on immediately, while her counter-terrorism-trained husband doesn't notice.)
 * In season 5 of the series, Jack is held hostage by a group of terrorists, and (rather unsubtly) relays a code he knew when he was employed by CTU ("I'm in a FLANK! TWO! POSITION!") to indicate he is transmitting under duress. This enables the strike teams to overtake the terrorists and save Jack... But only after McGill figures out the outdated code.
 * Used quite a bit on Knight Rider, usually to indicate something very wrong with KITT. In the episode "Killer KITT", KITT snapping at Michael was an indication that his programming was being messed with.
 * One episode of the new series had a hilarious variation when Mike is impersonating a member of a group out to steal from a Vegas casino. The leader figures out something is wrong because Mike is too competent.
 * Star Trek: Deep Space Nine:
 * In "Heart of Stone", Odo spends a lot of time with Changeling!Major Kira. He only realizes it is a double when "she" says "I love you".
 * In "Armageddon Game", an alien government has faked the deaths of O'Brien and Bashir (while trying to hunt down and actually kill them) in an attempt to destroy all knowledge of a dangerous bio-weapon. They send a recording of a faked lab accident to Deep Space Nine, using security footage from earlier on. But O'Brien's wife notices him drinking coffee, which he would NEVER do in the afternoon, and her suspicions along with the shady explanation of the accident itself is enough to get Sisko and the others to investigate further. At the end of the episode, O'Brien is recovering at home and asks for a cup of coffee after lunch, explaining to his shocked wife that he ALWAYS drinks coffee in the afternoons...
 * The Star Trek: The Original Series episode "What Are Little Girls Made Of". A robotic double is made of Kirk, with the robot having a duplicate of his mind as well. Kirk foils the plan during the procedure by mentally focusing on a racist insult toward Spock, something he would never normally say but which is consequently implanted into the double.
 * There's another one in "Whom the Gods Destroy," when the villain tries to bluff his way out of failing a Trust Password test by telling Scotty that he was just testing to make sure that he wouldn't let anyone beam up without the password. Given how long Kirk and Scotty have known one another, and how much they trust one another, Scotty immediately figures out that the "just testing you" story is bogus.
 * In Star Trek: The Next Generation episode "Allegiance", Picard is replaced by a double who has his memories but doesn't act like him very well--basically doing the kinds of things Picard would do if he weren't an incredibly reserved man (one of the weird things the doppelganger Picard does is go into Ten Forward, order drinks all around, and start singing). When the real Picard asks Riker what the giveaway was, he is told: "Well, sir, I find it hard to believe that you're that good a singer."
 * On Angel, Lorne appears to have pursued a career in show business and is only seen during brief telephone calls every few episodes, and each time, he asks how Fluffy is doing. The main characters are convinced that his success is just going to his head and that he is constantly snubbing them, but when they finally come to his rescue, he says, "Fluffy. Fluffy the dog. The dog you don't have. The universally recognized code for I'm being held prisoner. Send help!"
 * Hilariously played off when they got the call originally.

"Carter: "Besides, when was the last time you heard the Colonel use words like 'geomagnetic'?"
 * Also played with in the episode "Double or Nothing" when an old debt of Gunn's is called in (his soul) and he tries to push Fred away with some very mean words in order to spare her the pain of his leaving/death (not clear). Although visibly upset at first, she sees right through this ploy and tell the others he must be in some VERY BAD trouble because Gunn would never really say those things unless he was TRYING to push her away to protect her.
 * On Firefly, in River's backstory, she sends Simon and his family several odd notes referencing events that never happened, which actually contain a complex code intended to alert them to her mistreatment at the hands of the Alliance.
 * When Jack's smart house takes him hostage (along with several other characters) in Eureka, he responds to his deputy's concern with, "Sorry, false alarm. But thanks for coming down, Josephina." Since this is the only time he or anyone else has called her anything but "Jo," she realizes pretty quickly that something's out of the ordinary.
 * And in the season 2 finale, Taggart can tell that
 * NCIS, "Bete Noir": Kate attempts this when she brings a box of evidence down to the autopsy room on Abby's request only to find that Ducky has locked down the room and won't let anyone in. She pretends to be Abby, providing Ducky with the opportunity to confirm that something is amiss by playing along. Unfortunately the terrorist holding Ducky hostage realizes what they're doing, and Kate ends up a hostage as well.
 * In the episode "Bait", Gibbs is being held hostage by a teenager wearing a suicide bomber vest and demanding to see his mother. When the team finds out that the boy's mother is considered dead, Tony tells Gibbs that "Special Agent Caitlin Todd" is looking for her. When one of the local LEO's asks why Tony didn't reveal what the mother's death, he says he already has.
 * In Doctor Who, the Third Doctor once told the Brigadier to "tell it to the Marines" over the phone, alerting Lethbridge-Stewart to the danger.
 * In "The Android Invasion," the Fourth Doctor realizes Sarah Jane has been replaced when the android accepts the ginger pop he offers her, because he knows she hates ginger pop.
 * Actually he figures it out the moment he sees her, because, as he put it, "The real Sarah wasn't wearing a scarf." Him giving her ginger pop is just his way of checking to make sure he's right.
 * In "The Masque of Mandragora," the Doctor figures out that Sarah is under hypnotic control when she asks him how it is that she can understand the local language. He reveals this to her later (after he's broken the hypnosis), explaining that, "It's a Time Lord gift I allow you to share." (this is expanded upon further in the New Series, where it is explained that the TARDIS translates languages through the Doctor's head). The implication is that it's supposed to seem so natural that the companions don't question it. The fact that Sarah does question it is what alerts the Doctor to the fact that her mind is being controlled by an outside force.
 * Martha Jones figures out that her parents are speaking under duress when her mother tells her that her father is in the same room without so much as a raised voice.
 * Subverted in the same scene when Martha asks her father to just answer "yes" or "no" if there's someone else there. Unfortunately, the someone else can hear both sides of the conversation.
 * Inverted when the 10th Doctor spouts phrases against timey slimey clone Martha in "The Sontaran Strategem", as in "Avan ti!" instead of his usual "Allons y!" and a few times during the episode he can be seen eyeing her as a result of this.
 * Also done earlier in "New Earth", when Cassandra pulls a Grand Theft Me on Rose. He doesn't initially suspect anything, even when Rose suddenly kisses him and starts speaking a little strange. However, when they find out that and Rose doesn't react with horror, the Doctor knows something's wrong.
 * In this instance the Doctor starts to get suspicious just after she kisses him, when she leads them both into the secret intensive care unit by using technical knowledge he knows she doesn't have. Suspicions are confirmed when Rose doesn't care about
 * A variant is used in an episode of Stargate SG-1, where Daniel, trapped behind enemy lines, is relaying a battle plan to the SGC via radio through what sounds like small talk, mixing in Goa'uld words to let them know what his plan is. If the message was translated, it would sound something like, "Oh, and could you have Teal'c water my coordinated pincer attack?"
 * A straighter example would be when O'Neill and Teal'c are stuck in a time loop, but everyone else's memories are reset. O'Neill tries to prove it by explaining why the planet they're visiting is important before Carter has a chance to. Later, Carter and Hammond have the following conversation:

Hammond: "You have a point there."

Carter: "And he was using them correctly. More or less""

"Col O'Neill: Now, how did I know you were going to say that?
 * Except he didn't say "geomagnetic". He just said it has something to do with magnets. Teal'c was the one who said the correct term. Then again, O'Neill' isn't as dumb as he likes people to believe (besides other things, he's also an amateur astronomer).
 * Also from that episode:

Maj. Carter: Maybe you read my report?

Dr. Jackson: [skeptically] Maybe he read your report? [raises eyebrows]"

"Pilot: Ka D'Argo is currently... helping Rygel with his... laundry. And Aeryn's writing some... poetry.
 * In an episode of Charlie's Angels, one of the girls has been captured and is forced to act on the phone to the others as though everything is OK. She plays along, but drops in a reference to her station wagon, something the Angels are not known for driving.
 * Eleventh Hour, "Miracle": Patrick Stewart's character realizes that the apparent suicide of a doctor he'd been working with isn't what it seems when, in her suicide note, she bequeaths to him her "geiger counter". At an earlier meeting, she had made a point of her aversion to that term (on account of Hans Geiger's naziism), insisting instead upon calling it a "radiation detector".
 * Supernatural used this in Hunted. When Gordon uses Dean as bait and forces him to call Sam, Sam instantly knows that something's wrong because Dean used the codeword "Funkytown"..
 * Actually done several times. Once by a shape-shifter. Another time the body of another supernatural hunter was possessed by the Seven Deadly Sins.
 * Dark Angel uses this in the episode "Rising". When the villains tries to use Original Cindy as bait, she warns Max by referring to her new boyfriend. Original Cindy is a lesbian.
 * However, since Viewers are Morons, they had a earlier scene where she spent like five minutes listing some of her old girlfriends and how she would never ever ever date a man.
 * In the first season finale of Burn Notice, Sam provides a variant: in his proof-of-life photo, he makes a strange hand gesture. Michael looks through Sam's old photos and finds a matching pose, realizing that Sam is telling Michael to give up on him. Michael promptly ignores this and rescues Sam anyway.
 * In the third season finale, Madeline Westen apparently had a code phrase meaning "stay away" already established with her sons. It was originally meant to indicate that their father was in a drunken rage, but it also works pretty well when.
 * In the fourth season episode "Out of the Fire", Michael calls Fiona 'honey' while talking to her over the phone. She then explains that this a code phrase they had during their Belfast days meaning 'something is wrong'.
 * Subverted in the episode "Psirens" of Red Dwarf, when the crew is trying to figure out which of two Listers is a mind-reading impostor. The two act alike, until the crew has one of them play Lister's guitar. Because Lister sincerely (but utterly incorrectly) considers himself a brilliant player, the fake reads this, belts out a few power-chords, and goes down in a hail of laser-fire.
 * Played straight earlier when Dave realizes that the "Kryten" that saved him was not really Kryten, since Kryten never calls Lister "Dave".
 * On Psych, Lassiter gets a text message from "Shawn" and follows it right into the trap the killer set. When Lassiter gets there, a captive Shawn says, "I can't believe you thought that text was actually from me. It lacked all nuance, was lacking my signature mocking tone, and was totally devoid of emoticons."
 * Andromeda had Dylan Hunt blinking a code with his eyes when he was kidnapped, his ship computer was programmed to detect it.
 * In a recent episode of CSI, Riley calls Greg Sanders by her own name to alert him to the fact that she and Langston were being held hostage (a technique established at the beginning of the episode in a training roleplay). He replies "Okay, Sanders" to let her know he understands the message.
 * An episode of Tales from the Crypt had the bad guy threatening the protagonist with his wife's gun, which she normally keeps in her purse for self defence. With the husband taken hostage using the gun, he attempts to lure the wife in by telling her that the husband is threatening to commit suicide with the same gun he's holding. Unfortunately for the bad guy, both husband and wife know that
 * In a first-season episode of Jericho, Johnston uses this technique to flush out a group of desperate con-artists posing as Marines, by mixing Marine and Army mottos to see if they'd recognize the wrong ones.
 * On Farscape, episode "I Shrink, Therefore I am". Crichton is returning to Moya, which unbeknown to him has been hijacked. He messages the ship:

Crichton: Uh-huh. What about, ah... Chiana and Sikozu?

Pilot: Enjoying each other's company. Preparing a meal for... everyone but Rygel. He's... not hungry."


 * After hanging up Crichton says "something's wrong".
 * On Criminal Minds, Reid does this in the episode "Revelations" when he's being held hostage. The killer forces Reid, on a video feed, to pick a member of the team to die. Reid picks Hotch, and mentions a few character flaws and a relevant Bible verse. Hotch realizes that the personality Reid is describing isn't him -- it's a profile they had been discussing earlier. (To prove it, he has everyone on the team list his worst flaws, and none of them agree with what Reid said.) Hotch then looks up the Bible verse, and realizes that the quotation was incorrect. Since Reid has an eidetic memory, he would never quote something incorrectly. The actual passage is a clue to where Reid's being held.
 * In an earlier episode, Hotch was uncharacteristically frustrated with Reid's earlier difficulty in the shooting range after they had both been captured. He eventually convinced the UNSUB to let him kick Reid before they both died, which gave Reid a chance to grab the gun in his ankle holster. Later Reid said he'd figured out the plan at the very beginning, making the drawn-out scene unnecessary.
 * In the DVD commentary for "Revelations," the writers mused on this swap and eventually agreed that Reid and Hotch have "a very effective spite-based communication."
 * During the second season arc of Alias, Sydney and Jack's cover as agents of SD-6 is in jeopardy. Jack is in the custody of an SD-6 higher-up, ordered to bring his daughter in under suspicion of being moles. He calls Sydney, then tells her to take "surface roads," because of traffic. That's their code for, "we've been discovered, it's not safe."
 * In an episode of Beverly Hills, 90210, Donna is being held prisoner in her room by a would-be rapist. When David shows up at the apartment to apologize to her (they had an argument earlier in the day), the rapist orders her to get rid of him. So from behind a closed door, Donna repeatedly yells at David to leave, but she keeps calling him "Dave", something she's never done before. David realizes something's wrong and manages to save her.
 * In Dallas, Pam signals to Bobby that something is wrong by telling him on the phone that she plans to spend the evening playing backgammon with J.R. Bobby knows Pam and J.R. hate each other and would never willingly spend the evening together, so he hurries home to find the whole family is being held hostage.
 * Castle: Castle is being tied up and held at gunpoint by a killer when his mother calls. The killer forces him to answer the phone and act normal. When Castle ends the call with "I love you", his mother calls Beckett and tells her something must be terribly wrong.
 * in a previous episode, a 12 year old boy was being held hostage, and used the proof of life video his captors took to try to alert everyone to his location. " Don't forget to feed Ace," he says, only his family doesn't have any pets. Ace turns out to be a place, a subway station where lines A, C and E all run. Thanks to Castles quick thinking and the victim of the week's invention, the kid is rescued.
 * In a chilling episode of Rescue 911, a woman is told by a rapist to call in sick to her workplace. She promptly dials a male friend and tells him, "I can't come in to work today," thus alerting the friend that she's in trouble and he needs to stop by PRONTO. A few seconds later, she does the same thing again with 911, and the dispatcher immediately works out that there's something wrong and sends the police.
 * There was a similar episode of I Survived. When a woman's crazed ex-husband broke into her house and threatened her, she managed to stall him by telling him that her friend was coming to take her shopping and that she needed to cancel her plans. Amazingly, he let her call. When the friend answered, the woman proceeded to cheerfully tell her that she couldn't make it and not to bother coming. Initially confused--because they did NOT have plans to get together--the friend quickly realized that something was wrong and asked if the woman's husband was there. Upon being told "yes", the friend immediately called 911.
 * On an episode of Baywatch, as Stephanie gets into her truck, she's confronted by an escaped convict, who orders her to drive off the beach. During the drive, Mitch radios Stephanie to tell her to return to headquarters. The criminal orders Stephanie to tell Mitch that she has found a lost child and is driving him around to find his parents. Stephanie complies, knowing that the criminal has just hoisted himself by his own petard. Ironically, in his efforts to avoid detection, he's the one who has told her to say something she would never say. The lifeguards NEVER drive lost children around to find their parents. Policy dictates that they bring the children to headquarters. Sure enough, Mitch realizes Stephanie's in trouble and sends the police to find her.
 * Rather terrifyingly Inverted in an episode of Merlin. The setup is that Morgana has convinced King Uther that his son Arthur has been enchanted by Guinevere. Convinced that Arthur is under a spell Uther orders Guinevere to be burnt at the stake, at which point a panicking Arthur tells his father that he'd be willing to renounce his claim on the throne if only Guinevere is spared. Unfortunately, Uther takes this as "final proof" that Arthur is under a spell, claiming that it's something that he would never say. Except of course, he does.
 * Played straight in a later episode: Gaius is being controlled by a goblin. Arthur suspects as much and tricks the goblin into revealing itself by discussing Merlin's imminent execution. When goblin!Gaius does not show the least bit of a negative reaction to this, Arthur knows that Gaius is not himself, so to speak.
 * In the episode "Chris-Crossed" from Charmed, Chris is forcibly taken to the future, and tells the sisters right beforehand, "Looks like Leo's going to have to fix that floorboard without me." They initially misinterpret this to be his final words of defeat, as it seemingly has no relevance to the situation and he hates Leo, until they figure out Chris is trying to get them to put a power-restoring spell underneath the floorboard for him to use in the future.
 * Another Charmed example has Piper being possessed by a demon. Piper warns the demon that her sisters will eventually figure it out, "Or if they don't, my boyfriend Tom will." Piper's fiance (the aforementioned Leo) comes in just then and immediately becomes suspicious when the demon in Piper's body calls him Tom.
 * In the JAG episode "Secrets", Admiral Chegwidden is being held at gunpoint, and tells Bud over the phone to get him a specific file. Harm and Mac realize the file the Admiral asked for is about a sailor who held his CO hostage, tipping them off.
 * In Chuck versus The Santa Suit,  has taken over Castle and is holding Sarah hostage. In getting an important file, Chuck apparently gets a call from Sarah, but as he's able to deduce, it's just   using a voice modifier. How? Chuck concludes that Sarah would never call him "dear".
 * In the MacGyver episode "Countdown", Mac cues Pete in on the fact that he needs to speak to him on a private channel by 'reminding' him that they are due to play golf when he gets back. Mac has never played a round of golf in his life.
 * In an episode of Kaizoku Sentai Gokaiger, the Monster of the Week assumes the form of Yellow Ranger Luka. Near the end of the episode the rest of the team reveals that they were aware for some time, since at dinner the monster ate broccoli, a food Luka absolutely despises.
 * In Sherlock, John blinks SOS while he's strapped to the semtex vest and faces Sherlock after being taken hostage by Moriarty. There's no indication that Sherlock got it
 * On The X-Files, this happens with a twist. In the episode "Small Potatos," Mulder is kidnapped by a shape-shifter who decides to steal Mulder's identity...literally. In "Dreamland," Mulder and an MIB switch bodies. Both non-Mulders try their hand at seducing Scully. The MIB was so sleazy about it that some of the things he said were a big sign to Scully that Mulder was Not Himself. Plus, they both called her "Dana," which Mulder never does unless things are really bad.

Video Games

 * Super Robot Wars: Original Generation subverts this. When a main character's girlfriend disappears and later reappears, the character remarks that it's not her, because "even she wouldn't be silly at a time like this". When they break the mind control, she acts just like she did when she was mind-controlled. Another character mutters "Would never act like this in this situation, huh?".
 * In Marvel Ultimate Alliance, Doctor Doom's henchmen get "Dum Dum" Dugan to come to the S.H.I.E.L.D. Omega Base by relaying a Distress Call. However, Nick Fury figures out it is a trap when Dugan (purposefully) lists Bruce Banner as being a scientist researching the Super Soldier Serum when he is in fact assigned to the Gamma Bomb project.

Web Comics
"Faye: No mother, the peaches are definitely not ripe."
 * A Nodwick strip details a dungeon crawl where the group finds a high-up switch. The group is tipped off that something is amiss (viz. he's been replaced by a doppelganger) when Yeagar, looking for something to throw at it, ignores Nodwick in favor of a rock lying at his feet.
 * In Questionable Content, Faye and her Crazy Prepared mother have worked out a code phrase for when Faye was being held against her will.

"Belkar: Let's stop the violence.
 * Belkar of Order of the Stick subverts (double-subverts? parodies?) this during aftermath of the fight with the thieves guild. Celia and Hayley take it as Belkar gone Ax Crazy. More so than usual, anyway.

Side Character: See? Even your other party member agrees which means-

Celia and Haley: (together) RUN FOR YOUR LIVES!"

"Tarquin: Plus, he didn't rant about his intellectual superiority even ONCE.
 * Later, Tarquin figures out that Elan is not Nale partly because Elan surrendered to save a teammate, which Nale would never do.

Malack: Now that you mention it, I did find that odd."

""That letter is a fraud! Othar Tryggvassen may do things that lesser men find objectionable or slightly illegal, but I never apologize!""
 * Played with in Terror Island. In this strip, the protagonists recognize that their friend Jame is possessed by a demon, when he says "SOUNDS LIKE SOMEBODY HAS BEEN EATING SOUR GRAPES." But when the demon is banished from Jame, he says exactly the same thing, and Sid proclaims "That's our Jame!" Unless it's because demons simply talk with different Speech Bubbles...
 * In Othar Trygvassen's (Gentleman Adventurer) twitter:


 * In Something Positive, Eva had an Internet romance with someone who claimed to be Davan. Davan's friend Josh, however, was suspicious, partially because the e-mails all had perfect spelling, something Davan apparently doesn't bother with. Unfortunately for Eva, she didn't listen.
 * In Exiern Neils tries to drop hints here that he has been be-spelled and is no longer fully in control of his own actions by calling Crown Princess Peonie the "Second In Line To The Throne" instead of the first. It seems like no one notices at the time though. Peonie had to have it spelled out to her after the inevitable kidnapping, but her father was just going along with it 'til he could get his other assets in play.

Web Original

 * In the Whateley Universe story "Test Tube Babies", team superboy Lancer is fighting a power mimic/shapeshifter who now looks like Lancer. One of them yells at a teammate with an anti-brick weapon to 'shoot both of us'. The teammate blasts that one senseless. Not only is the teammate Genre Savvy, but they have communicators they would really use instead.

Western Animation
"Lisa: Dad? Where are you?
 * Parodied in The Simpsons, episode "Midnight Towboy", when Homer is kidnapped and calls his family.

Homer: (reading from a cue card held by the kidnapper, bad acting) Do not worry, I am working.

Lisa: Working? Where?

Homer: Ask no questions and do not call the police or I will kill me.

Lisa: You sound weird.

Homer: Everything is fine, goodbye forever. (Hangs up)"

"Terry: How did you know you weren't going crazy?
 * A variation of this occurs in the Sonic Sat AM series. Sally has been replaced with a robot duplicate, and says a multitude of things she would never say. It's when she replaces a "thumbs up" gesture with Sonic with a "thumbs down" gesture that Sonic gets it.
 * Another Western Sonic series, Adventures of Sonic the Hedgehog, also pulled off a variant of this trope. Sonic becomes concerned about Tails' safety, so after some false starts he thinks he's found a perfect family to adopt Tails, but then becomes depressed that Tails is gone. He has a flashback of the parents welcoming "Tails" to their family... and then suddenly remembers that he gave him the name Tails; Tails' real parents should have called him Miles, the name they gave him! Sonic rushes to the rescue; the "parents" turn out to be Robotnik's robotic goons in disguise.
 * All-around skewered in the South Park episode "Spookyfish", where "Evil Cartman" (actually a sweet and lovable Mirror Universe Cartman... from a MU where everyone has a goatee and is the opposite personality) and "Real Cartman" fight to avoid being sent back to the MU. When the goatee gets ripped off, one Cartman says they'll just have to send both back. Wanting "Evil Cartman" to stay, and knowing this is something "Real Cartman" would never say, the boys send the other Cartman back... except that Real Cartman is quite Genre Savvy...
 * In an episode of Batman: The Animated Series, the daughter of the mechanic who works on the Batmobile is kidnapped. With the bad guys listening in, Batman asks where she is, and the mechanic responds that she's "in the basement", which is apparently racetrack slang for in big trouble.
 * In an episode of Batman Beyond, Bruce realizes that his "inner voice" isn't really his, that he's under suggestion by the villain Shriek:

Bruce: The voice in my head kept calling me Bruce... I don't call myself 'Bruce'.

Terry: (in Batman voice) Don't forget, that's my name now...

Bruce: Tell that to my subconscious."

"Frylock: The real Shake thinks the blood drive is a pyramid scheme."
 * In a crossover episode, Robin calls Superman to Gotham, as Batman has gone missing. As proof that something is wrong, he shows a video message of Bruce Wayne calling Lucius Fox about going away for a while. When Supes asks what's wrong about that, Robin points out that Bruce chuckles at the end. Bruce Wayne never smiles, much less laughs. Turns out he's under Brainiac's mind control, but not because he's Batman (which Brainiac doesn't know) but because he needs access to Wayne Enterprises resources.
 * In The Spectacular Spider-Man  The Chameleon would though.
 * On an episode of Men in Black, J instantly realized that an alien was impersonating K, because K would never had laughed at any of his jokes.
 * On Teen Titans, the hero Jericho is possessing Cinderblock to get him and the other good guys into the bad guys' lair. However, he villains are immediately suspicious when "Cinderblock" says "Thank you" to one of their comments--having never fought these villains before, Jericho didn't realize that Cinderblock doesn't normally talk.
 * Which is sort of ironic, because usually, Jericho doesn't either.
 * This was how Cosgrove discredited an evil clone in Freakazoid: he asked if the clone wanted to go to a Yakov Smirnov festival. The clone said "no". (Compare that to Freakazoid's usual "DO I!")
 * In the opening movie of the Star Wars: The Clone Wars cartoon, Rex clues Anakin and Ahsoka into the fact that the surviving members of the group of clones they'd been captured by saying "Anakin, we've held the droids. What is your position?" This clues them in because all clones address Jedi by either "General" for Knights or "Commander" for Padawans, never by name.
 * Averted on Family Guy. Lois is being held at gunpoint by Diane Simmons, and she tries to signal to Peter that something by calling him "Pete" (something she never does), while Peter does question it, he quickly ignores it and leaves so he can listen to music in the car.
 * In Aqua Teen Hunger Force a (very degraded) clone of Shake was caught when Frylock suggested they give blood.


 * Subverted in American Dad. Francine is threatening Stan with a gun for trying to ditch her at her high school reunion with his CIA body double, Bill. One of them gives a heartfelt speech about how he was selfishly putting her down while trying to make up for his past inadequacies. Francine determines that the real Stan would never say something so sincere and shoots the opposite.

Real Life
""Dear XXX, let me assure you, all the stories about Jews having to suffer in Germany are nothing but propaganda. We are fine in every way, we are not harassed by the government, and we wouldn't wish to be anywhere else, except maybe with our dear aunt Sara - Sichrona la olam!" (The latter being a Hebrew phrase roughly equivalent to "May she rest in peace".)"
 * The crew of the USS Pueblo, captured by North Koreans, and their Hawaiian Good Luck Sign.
 * A captured Korean War commander, forced into a confession, gave this: "We paean the great state of North Korea and its leader." For those of you who didn't get it
 * Some biometric fingerprint scanners have a "panic" feature: One of the user's fingers is designated as the "panic finger": In normal conditions the user does not press it against the scanner to be verified; if they do, it still grants access but also trips a silent alarm.
 * As mentioned in Bravo Two Zero, people in the military, particularly special forces who may be likely to be captured, will often agree on a sign that they are being forced to do something under duress. In this book, one of the captured soldiers is forced to make a video message stating that all's well. He is given a cigarette to add to the illusion. His signal that all's not well is to hold the cigarette differently than he usually would.
 * Resistance radio operators for the British Special Operations Executive were given special code words to indicate if they were being forced to transmit under duress. Unfortunately some of these agents were later "reminded" by headquarters to include their security check when they omitted it after being captured.
 * Security checks, such as using codewords and including/not including certain punctuation characters in certain points of a message, to indicate that the operator now works in captivity, were the standard practice of radio operators who transmitted intelligence information from behind the enemy lines during WW II.
 * When captured during The Vietnam War, then-aviator Jeremiah Denton communicated to the American audience during a televised interview by blinking in Morse code, spelling out the word "torture".
 * One urban legend describes a prisoner of war forced to write a letter to his family saying that he is being treated well. In the letter is a cryptic sentence: "Please give little Jimmie the stamp for his collection." The family doesn't know anyone named Jimmie, so they realize that it must be a clue. They steam the stamp off the envelope, and on the other side is written the truth of the prisoner's condition: "They've cut off my [hands/legs/tongue]."
 * Then how did he-- oh, Urban Legend.
 * Not to mention that letters sent by POW don't have stamps, at least not before they've entered Red Cross' hands.
 * I heard of the inverse, a prisoner of war wrote to his parents who lived in a village small enough not to have street names. The fake address he made up (Kings' Road or Road of the Kings, plus a ridiculously high number) was a quote from The Bible (Book kings, the number indicating the verse) describing prisoners being treated WELL. By using a code to say this the family could be sure it was true.
 * Some businesses have, as a standard response to someone making threats, a protocol for calling 911. The employee tells the person they will have a supervisor paged, pick up the phone, dial 911, and then hang up. When 911 calls back, they respond as if talking to a supervisor with simple "yes" and "no" answers.
 * Arrested by the Nazis during WWII for suspicion of hiding Jews in her house, Corrie ten Boom received a letter from her sister with bad news (their father's death). She then noticed the address was written in a hand that sloped uncharacteristically forwards, pointing to the stamp. The stamp had the message "All the watches in your closet are safe", letting her know the hidden Jews had not been found by the Nazis.
 * They had that code already. "We have a woman's watch here that needs repairing. But I can't find a mainspring. Do you know who might have one?" was one way of saying that there was a woman in need of a hiding place, but none available. Any references to issues with a watch's face meant a Jew whose features were especially Semitic -- "Do you know someone willing to take on the extra risk?" And "This watch cannot be repaired -- do you have a receipt?" meant "Someone has died. We need a burial permit."
 * From a letter by a Jewish family which passed through Nazi censorship:


 * If the pilot of an aircraft sets their transponder code to 7500 or says 'squawk 7500' over the radio and then does not respond, air traffic control will assume the aircraft is being or has been hijacked. There are also other, confidential, measures taken. In fact, great emphasis is placed on the part of radio training where pilots are instructed how to avoid accidentally flipping their transponder to 7500 while switching codes.
 * At the National P.O.W. Museum (496 Cemetary Rd, Andersonville, GA), by the Andersonville civil war prison, there are a number of video screens to watch, including one screen showing black and white footage from WWII Japan. In this video there is a prisoner leaning on a railing, facing generally toward the camera. The man is unobtrusively giving the Hawaiian good luck symbol, so this gesture in film pre-dates the Korean war. Museum staff were unaware of this.
 * It's very obvious when someone else is doing something on another person's Facebook, or MSN.
 * There was a meme for a while of hacking, say, Bobby's facebook and making his status "hi my name is Bobby" as a signature.