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 LaFoote (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.
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 Trust 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 tell that when they found Ao he was being controlled by someone else's technique by the fact that he agreed when she said she would remove the seal around his Byakugan (when none of them could) and apologized to Chojuro (who he had previously been Drill Sergeant Nasty towards).
When Zetsu clones are infiltrating the allied forces in disguise, Sakura realizes Neji is one when he refers to Tonton as if she were a person, rather than Tsunade's pet pig.
This happens in Fushigi Yuugi when the mirror-Miaka tries to seduce Hotohori to overpower both him and Tamahome, while keeping the real Miaka trapped inside a Magic Mirror. Before she can kiss him, though, Hotohori lifts his sword to stop her. When the mirror-Miaka asks what's going on, Hotohori tells her that Miaka would never 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, Princess 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. Turns out it was all an act; everybody knows Hiruma would never tell them to give up unless the odds were exactly zero, so everyone (and we do mean everyone) silently went for an onside kick as to not let the enemy know they were changing plans.
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.
After Cell absorbs Android 17 in Dragon Ball Z, he tries to persuade 18 to "join" him by using 17's voice. As he talks about how amazing the power feels, 18 looks uncertain - but then he refers to their creator, Dr Gero, a little too glowingly. 18 snaps that she and 17 actually hated Gero for turning them into cyborgs and 17 would never care about his plans.
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 dear mom (Neo Queen Serenity aka Usagi's future self) and calls to her using the name "Rabbit". Chibi-Usa is at first fooled, happy to see her momma again, and runs to hug her... but she then suddenly stops and asks the monster "who are you?!" When the monster replies that she's her mother, Chibi-Usa says she's lying because her mother never calls her "Rabbit", which is whatthe Black Moon Clanrefers to her as. (Chibi-Usa's actual mommy uses "Small Lady" instead.) In the dub, this is reversed: "Small Lady" is what the Dark Moon calls her, and how she finds out her mother is a fake.
In the Sailor Moon SuperS Movie, Sailor Moon gets trapped in a dream world while trying to rescue an abducted Chibi-Usa. There she meets Mamoru, but suspects something wrong when Mamoru suggests she abandon her mission and her team to stay with him. Her suspicions are confirmed when she baits him by asking who he likes better: Her or Chibi-Usa- and he easily picks Usagi. This is a Call Back to earlier in the movie when real Mamoru could not answer this question since he was being asked to choose between his future wife and daughter, both of whom he loves dearly and deeply.
In Card Captor Sakura, Sakura was confused and hurt because the ghost of her mother Nadeshiko led her to a Death Trap (trying to drown her in the manga, getting her to walk off a cliff in the anime); She asks Yukito about it and he says that a mom would never want her child to be in danger. Later, when the "ghost" tries it again, Sakura remembers this and confronts it, saying "You're not my mom! TELL ME, WHO ARE YOU?!". It turns out it was a Clow Card, more exactly The Illusion, trying to avert being captured. The real Nadeshiko's ghost had actually SAVED Sakura from death via either pulling her out of the pool (manga) or slowing her fall enough for Yukito to catch her (anime).
In Fresh Pretty Cure!, Love Momozono's mom gets captured and duplicated by a monster. Setsuna, who by that point had been Happily Adopted into the Momozono family, figures it out first, but Love is fooled and gets in a fight with Setsuna because she thought Setsuna was being ungrateful to their mother. Later, Love figures it out because the monster-mother keeps mentioning Chiffon — who Love had never mentioned by name to her parents before. She then tests the fake by putting on a red bracelet that the real Mrs. Momozono had made for Setsuna; the monster-mother tells Love that it looks good on her, which confirms Love that the monster is a fake and alows her to properly confront her — her real mother would know that the red bracelet was made for Setsuna and there was a pink one for Love.
Saiyuki: When Chin Yisou creates a fake Goku, the team catches on almost immediately, and Sanzo promptly shoots it in the head. When Chin Yisou asks what gave it away, Gojyo explains that Goku would have asked for food the moment he showed up (though Sanzo's dialogue suggests he may have sensed something on a more instinctual level). The scene immediately cuts to the real Goku complaining that he's hungry.
K-On!: This is one of the things that blows Ui's cover when she's pretending to be her older sister Yui while the Light Music Club are practicing for the second school festival when the latter is sick. The whole club is fooled because of how close they look visually and Ui is really good at picking up things like her sister, so at first they practice without anyone the wiser. But then the band members notice "Yui" keeps time too well. In the ensuing discussion, she calls Azusa, Ritsu, and Tsumugi the wrong honorifics ("Azusa-chan", "Ritsu-san", and "Tsumugi-san"; Yui uses "Azu-nyan", "Ricchan", and "Mugi-chan"), and when challenged Ui could not produce Yui's nickname for Azusa.
Astute viewers can also catch "Yui" wearing the wrong color Inside Shoes when she first walks into the scene, something which all the members miss. Likely the others were so relieved at seeing "Yui" after missing several days they weren't paying attention.
In Natsume Yujinchou, everyone does notice Natsume is acting strange when Nyank-sensei impersonates him but assumes he just wasn't feeling well. However, knowing his involvement withyoukai, Tanuma does figure out it's not him and gets Sensei to reveal himself, much to Natsume's dismay.
Happens in Gunsmith Cats when Becky is being held at gunpoint to lure Rally into an ambush over the phone. Rally realizes there's a serious problem when she notices she didn't send Becky her fee and Becky made no comment whatsoever.
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 impostor 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.
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!"
In Faith and Doubt, it's revealed that Chrysalis' poor impersonation of Princess Cadance was due to the Princess acting like Prince Blueblood when she was caught, hoping it would clue others in. Chrysalis falls for it, but it fails to alert Celestia, due to her immense blind spot for her family.
The Twilight Child: Rarity notices something's up with Rainbow Dash when she starts enunciating properly. This is because Rainbow Dash has been on the receiving end of a "Freaky Friday" Flip.
In Sneakers, Crease draws Brice 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 Brice to leave with him by holding up the carphone and shouting that it's his mother calling. Since Brice's mother would have no way of calling him, and one of their group has the nickname "Mother," it's a clear signal that he needs to get out of there.
"Janelle": Wolfie's fine, honey, Wolfie's just fine. Where are you?
The Terminator:[hangs up the phone] Your foster parents are dead.
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 Director's Cut, however, John is tipped off when he sees his "mother's" foot melting into the floor.
The DVD release includes an amusing alternate take where Mal instead screams that last "TRAP!" at the top of his lungs.
Subverted in The Bourne Ultimatum, when Nikki is in a room with Jason Bourne and asked to "code in" on the phone to her superiors. Her personnel record is shown on screen, with responses for "normal" and "under duress", but despite being in a potentially dangerous situation she gives the "normal" response. This was because she'd switched sides.
And it becomes a Double Subversionbecause her superiors figure out Bourne is there anyway and send troops in to get him.
And both Bourne and Nikki know it, despite her giving the "normal" response.
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.
She also deliberately starts a fight with her ex-husband, mentioning a place seemingly from their past. Except it's a subtle historical reference that only her ex-husband and son (and her son's ex-girlfriend) could figure out.
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?"
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.
In the Dirty Harry movie Sudden Impact, when Harry gets his usual morning coffee, the café is being robbed; everyone is being forced to act naturally, but the waitress (knowing Harry's usual is "no sugar, no milk") adds lots of sugar. Harry, distracted by some case work he's going over, just pays and leaves with the cup. Outside, he takes a sip, spits it out and turns back to the diner.... then notices "patrons" locking the doors and turning the "Open/Closed" signs around, figures out what's up, and goes around to come in through the back.
In 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. Knowing that the cop can't see this, he then gives him the coffee for free. Sure enough. when the cop he goes outside to join his partner, he tells him that the man is being robbed—aside from the free coffee, having come to the store for years, they know that "that woman would show up for work even if she were dead".
Again, a "Something They Would Never Do"-Trope: in From Russia with Love, James Bond chastises himself for not recognising an enemy agent when the man orders the wrong wine with a fish course at dinner. Unfortunately, Bond only realises this hideous faux pas after the enemy has him at gunpoint.
In Superman II, the Krypton criminals led by General Zod force their way into the White House. They enter the Oval Office, and ask the person sitting behind the desk to kneel before Zod, which he does without saying a word. Immediately, they decide that he is not the President, since someone with that high a rank would put up some resistance.
In SurrogatesBruce Willis' character is tipped off that his partner's surrogate isn't being controlled by his actual partner when she refers to him by his last name. Everybody else in the movie does this, but she always called him Tom.
In Transcendence, Max quickly begins to suspect something's gone wrong with Will's upload when he's asking for access to financial and educational data almost immediately after he regains his sense of self. The Will he knows was never so proactive, which convinces him that this Will is just an echo superimposed over the PINN AI used as the base for his program. It turns out that Will's actions are entirely in-character, but now he has the means to achieve his goals instead of being content with limited gestures.
In the war film The Big Red One some German soldiers are posing as Americans. They're found out when somebody notices that throughout their meal they hold the fork in the left hand and the knife in the right instead of cutting off a piece of meat then transferring the fork to the right hand to lift the food to their mouth.
In Sherlock Holmes: A Game of Shadows, after an action sequence that separates the pair, Mary is understandably confused when Watson sends her a letter saying he doesn't miss her and hopes to never see her again. Mycroft Holmes tells her the truth can be found from a code that he and Sherlock devised as children, where a notice would be taken at face value or as the complete opposite depending on if the first letter of the note was a consonant or a vowel; in this case, Watson's letter is of course the complete opposite of what he means. The letter needed to be coded in case it were intercepted by someone affiliated with Moriarty, who had just targeted Mary in his scheme just to spite Sherlock Holmes.
How kid Xavier pierces kid Mystique's disguise at the beginning of X-Men: First Class. Well, that and the telepathy.
In Thor: The Dark World, a savvy viewer will realize early on that Loki's betrayal of Thor to the Dark Elves is an act. When doing so, he introduces himself as "Loki of Jotunheim". While he actually is a Jotun, he despises his birthplace and his birth parents, and whatever his issues with his adoptive family, he still considers himself to be Asgardian. Without that little hint, Loki's deception is extremely convincing.
In The Stranger, Franz Kindler (Orson Welles) is an escaped Nazi war criminal living in the United States under a fake identity. He's at a dinner party that also includes Mr. Wilson, a war crimes investigator who suspects that Welles's character might be the missing Kindler. A casual discussion of the German character turns to Karl Marx, which leads Kindler to respond by saying "but Marx wasn't a German; Marx was a Jew." This causes Wilson to realize that "Charles Rankin" is in fact the man he's been hunting.
Animorphs has a straight example when Jake is infested. His friends are convinced he has been body snatched when he shows sudden, inexplicable revulsion to Ax's touch 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 trying to weasel out of having to stay in the cabin for three days to starve out the Yeerk. It's pointed out that the real Jake would have gone along with the plan, no matter the potential for disaster, just to make absolutely sure that there was no Yeerk. At that point, the Yeerk really knows he's screwed.
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.
Again with another Enid Blyton story: in "Five Find Outers and Dog" Frederick Algernon "Fatty" Trottville is made to sign a letter by people holding him hostage. He is forced to sign as "Freddie" which makes Bets and them all suspicious.
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 & 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.
In one of the Wraith Squadron books, Face, undercover as a stormtrooper so as to steal TIE fighters for their pirate facade, has to speak the password to the supervisor (because the on-duty officer who knows the password can't say it or it would reveal something was wrong, and the one who was supposed to say it was killed during the break-in). He gets away with it thanks to his acting skills and the static disguising his voice...until he makes the mistake of referring (out of habit) to the TIE fighters as "eyeballs" (New Republic slang). He tries to salvage it by claiming it was a joke, to no avail.
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 girlfriend 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 -or 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 teacherMr. 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!" This ends up being a parody, as not only does she figure out that something is wrong from this, she somehow figures out exactly what inconceivably bizarre trouble they're in (that a three-nostrilled man was sucking their voices into his nose) and how to save the day. Just from someone being out of character.
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's 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 Op Center novel Games of State, a character is forced to send a message back to their Mission Control as part of the villains' plan. As is explained later, he had a pre-existing arrangement where every message he sent would include either a smiley facenote :) or a frownie facenote :( depending on whether he was fine or under duress. In this case, it is treated as a characteristic bit of paranoia that had eventually paid off, rather than anything that an agent would normally do.
In the Dresden Files novel 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 intentionally playing up being helpless, as it went against both the nature of their friendship and her strong, assertive 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, Nancy doesn't realize this and thinks the girl is trying to say she's okay, and as such, doesn't relay the message 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.
It was more of an Up to Eleven message as the friend had once said in a situation that looked like they might die, "Don't worry. Everything's going to be alright." The friend later stated that it would take a situation much worse to get him to say it again.
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. Or, errors she might have made as a coded signal that all was not well.Or, errors she might have made due to disruptions in her training which were caused by constant moving of the V.F.D. Headquarters.
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.
The gamebook The Mystery of Atlantis: If you try and tell a suspicious Athens city guard that you are a Scyth, he will promptly quiz you on a piece of Scythian culture (since he is a Scyth himself.) You fail ("What does a Scyth warrior keep tied to his horse's bridle?" No, not flowers. The scalps of his enemies), at which point he decides you're a runaway slave.
Several characters in Frederick Forsyth books use this when communicating from behind enemy lines. One trick is to always include a line of very slightly misquoted poetry - if a poem's quote is correct, it means he is operating under duress.
In Diane Duane and Peter Morwood's High Moon, the bad guys fake a message from the protagonists' superior officer to get them out of the way. They're briefly deceived, but then notice that it's signed in a nonstandard way, the routing makes no sense, and most importantly that this message from their cost-focused boss doesn't say a thing about the expensive reward they just authorized.
In Gaunt's Ghosts, the resistance on Chaos-occupied Geron respond to the code phrase "Geron survives" with "Despite their efforts" normally, while under duress they say "Even though it dies."
In the Alistair Maclean spy novel The Dark Crusader (also known as The Black Shrike) the hero includes the nonsense word 'Bilex' in all communications with headquarters, to show that he is not sending under duress. When he is captured, the bad guys tell him that they have sent a message assuring HQ that all is well. At the end of the book he realises that either this message (minus the safety word) was never sent or it was ignored by his superior. Either way, it proved that the superior was actually in league with the bad guys.
Live Action TV
In Smallville, when Clark's on red kryptonite, he always refers to himself as "Kal". Although his offensive and sexually aggressive attitude should tip you off quick.
Lionel in Clark's body has cruel comments for both Lana and Chloe, but the most obvious alert comes when he referred to Chloe as "Miss Sullivan".
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.
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".
Subverted 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 sure, he drinks coffee in the afternoon sometimes...
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."
In "The Most Toys", Kivas Fajo almost perfectly faked Data's death. He messed up in one key area, though; Data's final transmission, fabricated by him, was phrased incorrectly. The crew had been ferrying materials between their two ships, and Data had robotically repeated the exact same status report each time. The last one was missing the notification that he was launching the shuttle—Data had been incapacitated before the shuttle launch.
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 as they wonder if it's a pet nickname for a member of the crew, and which member that is.
Fred: "You don't think he was referring to anything of mine that's fluffy do you? Because that would just be inappropriate..."
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. Unfortunately Simon is the only one who can understand it.
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 the Eureka computer is faking Fargo's voice because it calls him "Teg", something Jo called him.
"Bęte Noire". 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 the mother's death, he says he already has. Special Agent Caitlin Todd is dead.
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. In fact 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 clone Martha in "The Sontaran Strategem", as in "Avanti!" 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 the hospital is making clones and infecting them with diseases and Rose doesn't react with horror, the Doctor knows something's wrong. In this instance the Doctor likely knew all along that something was wrong from all the clues she dropped but assumed that the problem was related to the strange hospital he was investigating rather than a separate entity coincidentally appearing. He mostly plays along with the ruse to keep things going and only openly admits he knows something is wrong when he has exposed the hospital's secret and is talking to someone with answers.
In "The Time Monster", the Master impersonates the Brigadier in a telephone call to Sergeant Benton. He imitates his voice perfectly but Benton is not fooled because brigadiers don't call sergeants "my dear fellow".
A variant is used in an episode 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:
Carter: Besides, when was the last time you heard the Colonel use words like "geomagnetic"? Hammond: You have a point there. Carter: And he was using them correctly. More or less.
Also from that episode:
Col O'Neill: Now, how did I know you were going to say that? Maj. Carter: Maybe you read my report? Dr. Jackson: (skeptically) Maybe he read your report? (raises eyebrows)
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". Subverted: Gordon knew that Dean would find a way to alert Sam and was leading him into a trap.
Actually done several times. Once by a shape-shifter. Another time the body of another supernatural hunter was possessed by the Seven Deadly Sins. In "The Great Escapist", Kevin Tran realizes that two people who look and sound just like Sam and Dean are impersonators because they are being too patient and helpful. The irony that a pair of demons would be too nice to convincingly pass as the heroes does not go unnoticed.
A plot point in season six, when Sam has been behaving a littleout of character. Turns out his body had no soul in it. Played with because it took Dean a third of a season to notice, so this trope was invoked for viewers as much as characters. What caused Dean to realize something was up was Sam letting Dean get turned into a vampire, as Dean saw that Sam didn't seem bothered by it and was actually smiling rather than trying to save him.
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 included an 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 the FBI is after Mike.
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".
In series one, Rimmer once tried to trick Lister out of taking a test that would have made him Rimmer's superior, and allowed him to replace Rimmer with a hologram of his crush Kochanski. He does this by taking on Kochanski's appearance, and telling Lister that she would never be interested in him. However, his performance is pretty lousy; "Kochanski" repeats some phrases Rimmer often uses, is easily bluffed when Lister mentions a fake tryst they had, and then tries to cover up her weird behavior by saying she's "having a woman's period".
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."
In a later episode, Shawn has been kidnapped. He calls Juliet, ostensibly to tell her that he believes he's going to die and wanted to talk to her one last time. He ends the call by saying, "Goodbye, Abigail," cluing her in to the fact that there was a hidden message in what he said. She can't figure it out, but she repeats the conversation to Shawn's father, who does.
Andromeda had Dylan Hunt blinking a code with his eyes when he was kidnapped, his ship computer was programmed to detect it.
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 the wife doesn't keep any bullets in the gun.
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:
Pilot: Ka D'Argo is currently... helping Rygel with his... laundry. And Aeryn's writing some... poetry.
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."
Sydney realizes that the fake Francie is an impostor when she offers her to some coffee ice cream, something the real Francie hated, and she accepts.
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, "3XK": 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 later episode, "Murder Most Fowl", 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 Castle's quick thinking and the victim of the week's invention, the kid is rescued.
Beckett is forced at gunpoint to call Castle and set up a meeting. Castle apologizes for a fight they had earlier about a dinner with their parents that went badly, and Beckett suggests that they should go to a baseball game because it's "something all four of us can enjoy". Castle recognizes this as a distress signal, since the cause of the fight was his mother disparaging Beckett's father's love of baseball.
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—the lifeguards NEVER drive lost children around to find their parents. Policy dictates that they bring the children to headquarters. The criminal's efforts to avoid detection have resulted in him being found out—Mitch instantly 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.
Similar to the Angel episode with Fred and Gunn, when Arthur breaks up with Guenevere in season 4 on the advice of Agravaine, Gwen immediately asks if someone talked him into it and she doesn't buy for a second that he made the choice on his own.
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.
A demon once tried to impersonate Cole, but made the mistake of telling Phoebe the safest place he knew was in the mausoleum. Cole's real "safest place" is with her.
In the JAG episode "Secrets" in season 2, 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, Shaw 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 Shaw 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.
On All My Children, Julia has been kidnapped by her recently paroled rapist. He orders her to call her boyfriend and tell him everything's okay. When she does, she tells him that she's assisting a woman from her rape survivor group, a "Mrs. Elijah". Elijah is the name of her boyfriend's former neighbor. He instantly realizes that she's being held prisoner in his old apartment.
Similarly, on One Life to Live, when Rebecca is kidnapped by escaped rapist Todd, she manages to tip her friends off to where she's being held (he lets her make one phone call to assure them that she's all right) by referring to a Bible verse that they had been discussing shortly before she was abducted.
Also on One Life to Live. Andy and Antonio have gone to New York for a romantic weekend, unaware that they are being stalked by a hitman who's pretended to befriend them. When Andy figures out what's going on, she quickly reminds Antonio that he needs to call his sister so that they can have an excuse to get away without arousing the hitman's suspicions. Antonio doesn't have a sister. Unfortunately, his brief moment of confusion before he realizes what Andy's trying to do tips the gunman off to the fact that they're on to him.
On General Hospital, when Mac was kidnapped and replaced by a double, he fed him incorrect information about his personal life, resulting in him treating his fiancee like dirt and basically sexually harassing his coworker who had no interest in him.
On Family Matters, Urkell was being threatened by a criminal causing him to be placed in witness protection, guarded day and night by, among others, Carl. At the end of his shift guarding Urkell, Carl's partner shows up, holding a large handkerchief in front of his face, telling Carl he has a cold. Carl's suspicions aren't raised until he says "see you later" and the criminal replies "yeah, see you" instead of the actual partner's trademark "not if I see you first". Carl then mentions some money he owes him, slides it under the door, then kicks the door down when the crim bends down to pick it up.
In the Mighty Morphin' Power Rangers episode "The Wannabe Ranger", Jason is able to see through the Primator's impersonation of Tommy when the monster suggests they give up, something the real Tommy would never say.
The Primator screws up again when impersonating Trini. The Rangers have trouble distinguishing between the real one and the fake at first, so Jason suggests that both of them fight him, so he'd be able to tell by both of their styles who's real and who's not. But as Jason was counting on, the real tip-off is that the real Trini refuses to fight him at all because he's her friend, while Primator disguised as Trini enthusiastically jumps at Jason with daggers poised to strike. Jason promptly orders the other rangers to shoot Primator down.
During Power Rangers in Space the villain Astronema uses a monster's disguise ability to disguise herself as the Yellow Ranger, during the episode, the real Yellow Ranger returns to help the team, but loses her Morpher, which the Red Ranger retrieves and then the Red Ranger has to figure out which one is the real one, how does he do so, only the Rangers would know how to activate the Morphers, thus, he asked them both, and Astronema didn't know.
One episode of Elementary has an example that refers back to a Chekhov's Lecture during the Cold Open. Namely, Watson complaining she can't read Holmes' texts because he uses too many weird abbreviations. After the Killer of the Week kidnaps Holmes, she sends a text to Watson from his phone so she won't worry. Watson recognizes it as fake because it didn't read "like a teenager on a sugar high." After Holmes is rescued, he tries to take credit by claiming he deliberately provoked the killer to text Watson.
In one episode of Flashpoint, Parker was held hostage by an ex-cop who had tried to join SRU but failed the psych test, causing his life to spiral down. The ex-cop told Parker to misled his team to a certain room, which Parker complied but added "stay frosty" alerting his team that he was under duress. However, the ex-cop realized it was a distress code when he was watching the security cameras and noticed the team wasn't heading towards the given room.
In one episode of Teen Wolf, Stiles, Scott and Allison have to kidnap Jackson. To throw off suspicion, Stiles sends a text from Jackson's phone to his father that says: "Stayed at friend's house last night. Everything fine. Love you." Jackson's father immediately knows that something is wrong, because Jackson hasn't been able to tell his parents that he loves them since they told him he was adopted, eleven years ago.
In the pilot of the Flash Gordon TV series, Flash receives a call from his mother asking him to come home. The viewer is shown his mother being obviously mind-controlled by an alien cyborg in the room. Flash realizes something is wrong when his mother calls him "Flash", which is a nickname that his father gave him, which his mother never uses, instead calling him by his real name Steven. He does come home, but he's planning for a trap.
Inverted on Without a Trace. An abducted woman is forced to call her husband and assure him that she's fine and that's she's merely hanging out with an old friend, rather than being kept there by force. Not believing her claims of being all right, her worried husband tells her to ask about their children if she's in fact being held prisoner. When she manages to cheerfully ask how the kids are, he realizes she's in trouble.
In an episode of Alfred Hitchcock Presents, an escaped convict holds a woman hostage in her house when the telephone rings. He makes her answer it, and, listening into the conversation, tells her exactly how to answer each question from the policeman on the other end. Later, after the guy let his guard down, the cops come busting in and arrest him. It turns out the woman was deaf but could speak without an accent and read lips as well. She was able to repeat what the guy said, but the cop on the phone knew she wouldn't be able to respond to his questions normally on the telephone.
Patricia being polite over the phone was enough to let everyone know that she wasn't alone.
Fabian taunting Joy is what made Alfie realize Fabian was a sinner. And whenever he's upset, you know things aren't going well.
On ER, as Sam is supposedly escorting a patient and two attendants to an ambulance, she calls Abby "Abigail", something that she (nor anyone else) has ever done. In reality, Sam is being abducted by the trio—they're her sleazy ex-husband and his two accomplices. Abby's confused, but she manages to realize that something's wrong.
A variant in the last season Buffy the Vampire Slayer: Willow sees a vision of Cassie, a girl who had died earlier in the season. "Cassie" explains that she's been sent by Willow's dead girlfriend Tara, who can't come herself because Willow's being punished for her Roaring Rampage of Revenge against Tara's murderer. At the end of the conversation, "Cassie" suggests that Willow kill herself so that she and Tara can be together again. Willow instantly realizes that Tara wouldn't send anyone who would suggest that, and the illusion, sensing its mistake, drops any pretense and reveals itself as the season's Big Bad, the First Evil.
The Quantum Leap episode "The Boogieman" has numerous in regards to Al: He's dressed in a conservative suit, his handlink never lights up nor does he ever really use it, he's never seen walking through any objects, and he never opens the Imaging Chamber door. It's finally revealed in the final act that he's been Satan in disguise.
On Justified Boyd Crowder calls up his new business partner, Hot-Rod Dunham to finalize the details of a major drug deal. At the end of the conversation, the Hot-Rod tells Boyd that he hopes everything goes as smoothly as the last time he, Boyd and Dickie Bennett did a drug deal together. Boyd instantly realizes that Hot-Rod is under duress and the drug deal is a trap. The last time Hot-Rod bought drugs from Dickie Bennett, things went far from "smooth" because Boyd ambushed them and stole Dickie's money. This was followed by a Mob War that saw many people die.
In an episode of Monk, Adrian realizes that a man who claims to be the brother of a missing person isn't really his brother, since they used different terms for soft drinks (pop vs. soda).
On Laverne and Shirley, when Shirley needs help after being handcuffed to an escaped bank robber, she calls Mr. DeFazio at his restaurant requesting "extra BBQ sauce" on her chicken. Naturally, Mr. DeFazio fails to note Shirley's "code"... although Laverne immediately understands that Shirley would never order extra BBQ sauce, because it would get under her fingernails.
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?".
During the attack on the Omega Missile base in Super Robot Wars Compact 3, Folka then attacks Fernando and demands to know what is wrong with Fernando since Fernando isn't usually this kill happy but Fernando says what's wrong with that, all Shura are born to fight and kill and strive for the position of General. Folka wonders what changed Fernando like this but Fernando tells him to shut up and fight and die. After beating him, Fernando then says, "What a redundant moron" which perks Folka's ear and Folka says that he won't be hesitant anymore, he'll defeat Fernando and Fernando says that Folka can try. Folka then smirks and says "How about revealing your true face... You can't fool my eyes." "Fernando" then asks how Folka knew that he wasn't really Fernando and Folka says that he hasn't forgotten Fernando's fist (The way of the martial arts, fighter recognize, communicate, etc with their fist) "Fernando" reveals himself to be Arco who says that he didn't think Folka had gotten this strong and Folka says that the power of belief has made him stronger. Arco then says that when the REAL Fernando awakens then Folka wouldn't be talking big like this and Arco notes that this is it for him, looks like what that girl said was true.
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.
In Star Control II, when you meet the final remaining Shofixti warrior, his sensors are nonfunctional and he assumes that you are an Ur-Quan destroyer sent to hunt him down. The only way to convince him that you are friendly, counterintuitively, is to insult him repeatedly. The Ur-Quan, for all their faults, are always very polite, even when preparing to kill an enemy. Eventually he realizes this and accepts that you are not an Ur-Quan.
In The King of Fighters, for all of her massive flaws Rose Berstein loves her older brother Adel and always tells him what's on her mind. When Adel finds out that she has become the new host for the KOF tournament and has built a whole new and lavish stadium for itbehind his back, he's extremely disturbed and thinks something's very wrong. He's right: Rose has been brainwashed by Those of the Past and is their Unwitting Pawn.
The last mission of the second week in The World Ends with You is ever-so-slightly inconsistent with the missions issued earlier in the week, as this one did not have any math-related puns in the message. Neku and Joshua notice this, but as it's still an official mission and the alternative is being erased, they do it anyway. Turns out it wasn't issued by the Game Master, but by the Conductor, who had decided the GM had gone rogue and needed to be eliminated.
One comes up in Tales of Symphonia's sequel, Dawn of the New World; when Emil, Marta and Zelos come across two Lloyd Irvings fighting, both Lloyds urge them to believe they are the real Lloyd and to strike down the other. The player is given the choice to decide who is the real Lloyd after hearing each one give their speech. The fake is the Lloyd who declared the fight was for justice, which is something the real Lloyd has vehemently said in the original Tales of Symphonia was a word he hated.
Faye: No mother, the peaches are definitely not ripe.
Belkar of The Order of the Stick... uh... subverts? (double-subverts? parodies?) this during the aftermath of a fight with the thieves guild. Celia and Hayley interpret it as Belkar gone... well, more insane than usual, since "usual" means "kill everything not allied to me."
Belkar: Let's stop the violence.
Side Character: See? Even your other party member agrees which means-
Nale: Did you ever think that maybe I'm tired of fighting, and that I just want to enjoy one day with my family?
Elan: No, that—that's not how you would react to this. That's how I would want you to react. At the very least, you should be trying to kill me for being chosen Best Man over you!
Later in the story, when the High Priest of Hel casts Control Weather, Belkar seizes on this as proof that he is not Durkon, as Durkon wouldn't need to prepare the spell, much less use it to stop a storm. He could simply pray to Thor, Durkon's patron. Unfortunately, no one believes him.
"That letter is a fraud! Othar Tryggvassen may do things that lesser men find objectionable or slightly illegal, but I never apologize!"
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.
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.
One episode of Atop the Fourth Wall has 90s Kid suggesting a Nirvana reunion that involves digging up Kurt Cobain's body and having zombie chicks play him like a puppet. It isn't until later, but it finally dawns on Linkara that not even 90s Kid is that twisted, leading him to realize he's been possessed by The Entity
In Worm, one of these acts as a Wham Line in Chapter 14.8, when Bonesaw is pretending to be Tattletale in order to gain Skitter's trust.
During the first episode of the Yellow Temperance mini-arc in Vaguely Recalling JoJo, Jotaro realizes that Kakyoin is a fake after witnessing Kakyoin in the act of devouring some beetles. Jotaro punches the fake Kakyoin in the face, which gets him to reveal himself as Rubber Soul.
This is pretty much what happened in the arc it's parodying, oddly enough. It helped that Rubber Soul is a jackass who doesn't bother trying to mimic his target; he likes making them look like sudden assholes.
Parodied in The Simpsons, episode "Midnight Towboy", when Homer is kidnapped and calls his family.
Lisa: Dad? Where are you?
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)
In "Futurama", episode Law and Oracle, Fry is able to tell that Pickles is deceiving him when he shows Bender sharing strong malt liquor to the crew of the Planet Express (which, though harmless to robots, is deadly to humans). As Bender is a Unsympathetic Comedy Protagonist, he would not share it with anyone.
A variation of this occurs in the Sonic SATAM series. Sally has been replaced with a robot duplicate, and the robot 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 finding him a family to adopt him, he finds what he thinks are his real parents. He leaves Tails with them, but then becomes depressed that he's 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 to be sure. Wanting "Evil Cartman" to stay, and knowing the "Real Cartman" would never be selfless enough to suggest such a thing, the boys send the other Cartman back... except that Real Cartman is Genre Savvy enough to anticipate this and trick them.
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 "Mudslide", a young security guard caught what he thought was a thief stealing his boss' safe, until he saw his boss telling him to get back to his post. The guard was about to leave when he noticed his boss's accent is gone and remembered he was on vacation in Hawaii. The "boss" knocked him out by throwing clay at him.
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 kept calling me "Bruce." In my mind, that's not what I call myself.
Terry: What do you call yourself? *Bruce gives him a pointed look* Oh, yeah. I suppose you would. (in Batman voice) But that's my name now.
Bruce: Tell that to my subconscious.
In Knight Time, Superman arrives in Gotham, as Batman has gone missing. During the investigation, Robin shows a video message of Bruce Wayne calling Lucius Fox about going away for a while. When Supes asks what's wrong with it, 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.
On an episode of Men In Black, J instantly realized that a robot 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.
When Madame Rouge is impersonating Robin to fool Hotspot, she repeatedly goes on about how Madame Rouge is the most dangerous foe he's ever faced. Pretty much anyone on the main team would know that's not true (It's Slade, obviously) but Hotspot is only an honorary member and didn't know this. Likewise she repeatedly gets Starfire's name wrong, and is getting increasingly agitated due to Hotspot's powers (since it hurts Rouge, even at a distance). Hotspot still doesn't catch on, however despite this he doesn't end up following "Robin's" instructions anyway due to pure stubborness about how he doesn't take orders from him, forcing Rouge to try to trick him by setting up an ambush instead.
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 have 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 is wrong 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.
Frylock: The real Master Shake thinks the blood drive is a pyramid scheme. Perpetrated by Dracula and his night slaves.
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 who, reverting to his Southern accent, turns out to be Bill the double, after all. Stan really was apologizing.
In one episode of Code Lyoko, Xana traps the team (except Jeremy) in a virtual version of the real world. Jeremy enters Lyoko to rescue his friends, and is confronted by an evil, virtual version of himself. Each tries to convince Odd, Ulrich, and Umi that they are the real one. Evil Jeremy gives himself away when he says he would "never go into Lyoko", but they know that the real Jeremy WOULD go into Lyoko if it meant rescuing his friends.
Played straight and inverted at the same time in a later episode, where XANA sends a polymorph clone to manipulate the Lyoko-Warriors by using their feelings against each other. At one point, the clone, while impersonating Jeremy, kisses Aelita to prove he is the real Jeremy, which ironically fails since she knew the real Jeremy was too shy to ever dare do it. Later in the same episode, Odd is facing the real Yumi and the clone in Yumi's form at the same time, and actually recognizes which one is real when the real Yumi insults him, because "XANA respects him too much to insult him."
In another episode XANA is pretending to be Franz Hopper. He gives himself away when he calls the Skidbladnir an odd name for a ship, when the real Franz Hopper was well versed in Norse Mythology.
On Codename: Kids Next Door, there's a parody that then switches to being played straight. In the P.O.O.L episode, all of Numbuh Four's friends get replaced by doubles from a negative universe. He doesn't catch on at first, even when negative Numbuh One is acting wildly out of character, being cowardly to Lizzie and even revealing some of the evil plan. That's the "parody" part. The "playing straight" part comes into play because Numbuh Four does eventually figure out the switch...because of negative Numbuh Three's slip-up, not negative Numbuh One's. Specifically, negative Numbuh Three has a mean and nasty look in her eyes, which Numbuh Four knows, as her friend, is something that would never happen with the real, sweet-hearted Numbuh Three (even when Numbuh Three gets angry it's out of hurt feelings and not cruelty). Ergo, Numbuh Four correctly concludes, they must be from a negative universe or something!
In another episode, when Numbah One is trapped in a Lotus-Eater Machine, he sees through the ruse when he notices Numbah Four swimming in a pool. The real Numbah Four can't swim.
In the season 2 finale of My Little Pony: Friendship Is Magic, Twilight realizes that Princess Cadence is not herself when she doesn't respond to her secret handshake and becomes even more suspicious when she sees Cadence acting unusually rude and selfish.
Later on, Twilight encounters the real Princess Cadence who proves her identity with the aforementioned secret handshake. The two race back to the wedding in time to expose the impostor.
The best way to free someone of Purple Man's control in The Avengers: Earth's Mightiest Heroes is to point out to the victim that they themselves are doing or saying something they would never do. Cap is freed by pointing out that he doesn't believe in forcing things on others, Hawkeye is freed by pointing out that Hawkeye never blindly obeys anyone, not even his superiors, Ms Marvel is freed when it's pointed out that she'd never carelessly ignore her allies in distress etc.
When the Winx Club search for the Gem of Self-Confidence, Bloom, Stella and Aisha find themselves facing illusions based on their inner fears. Aisha faces an illusion of Nabu, and quickly realizes it's not really him when the illusion blames her for his death, which is something the real Nabu would never do.
In the Goof Troop episode "Frankengoof", Frankengoof's monster looks very similar to Pete in Halloween makeup; so similar that his family initially mistakes it for him. Eventually they realize that the monster isn't really Pete because he's nice and willing to take responsibility, while the real Pete is a Jerkass and a Lazy Husband. The episode ends with his family deliberately abandoning him and bringing the monster home instead.
In an episode of The Transformers, Megatron builds a clone of Optimus Prime to lure the Autobots to their doom. Just when he manages to convince the others that he's the real thing, they get word that Spike is in trouble. Megatron, trying to keep his plans on track, says that Spike is unimportant, cluing the others in that he's fake.
In an episode of Transformers Prime Bulkhead noticed something off about his "friend" Wheeljack and asked him to tell the story of a particular battle. Makeshift, the impostor tells the story perfectly, but manages to get one crucial detail wrong... Bulkhead wasn't part of that battle.
In the Darkwing Duck episode "Battle of the Brain Teasers", Launchpad initially doesn't believe Gosalyn when she informs him that hat-shaped aliens that have taken over the bodies of Drake Mallard and Honker, even after seeing them with the hats. It's not until the hat that's taken over Drake has him change into his Darkwing Duck outfit - without his regular hat - that Launchpad becomes convinced, because the hat-shaped alien doesn't match the outfit and "DW would never wear a hat that clashes with the rest of his outfit".
Spoofed in Sev Trek: Pus in Boots, an Australian parody of Star Trek: The Next Generation. Captain Pinchhard (Picard), Cmdr Piker (Riker), Gaudy (Geordi LaForge) and Beta (Data) are hunting a shapeshifting alien that's roaming loose on the Enterforaprize. They enter a room to find Measley Cruncher (Wesley Crusher) who's just invented a Plot Device which will detect the alien and save the Enterforaprize for the 47th time! Captain Pinchhard gives a Big "NO!" and disintegrates Measley on the spot.
Piker:Oh my God, you killed Measley! [All point their phizzer rifles at the captain] Only the alien would do that!
Mystery Inc.: Freddy and Daphne get impersonated. While Daphne's fake is at least a little believable, Freddie's fake is a textbook example. The rest of the gang didn't really need the secret clue.
In Super Friends: The Legendary Super Powers Show, three Super Friends infiltrate an Auction of Evil to get a piece of Gold Kryptonite from Darkseid by disguising themselves as ice aliens. However, in the auction, Darkseid notices that those "ice aliens" are strangely comfortable at the station's normal room temperature and realizes they are impostors and exposes them.
In one episode of Max Steel, John Dread hacks into Max's coms signal and replaces Berto Martinez as Max's Mission Control. Max figures this out due to Dread referring to Max as "hombre" and "amigo", in contrast to Martinez's standard term of affection, "hermano".
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 paean is pronounced like pee on.
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 (allowing the user to discreetly summon help if he is being coerced into opening the scanner lock).
Similarly, password-based systems may include a "duress code" that triggers a remote alarm when entered.
There is a persistent urban legend that if a mugger tries to force you to withdraw money from an ATM, typing your PIN backwards will let the authorities know you are being forced to do it. Contrary to popular belief, while this system has been seriously proposed (and endorsed by many police officers), it has never been implemented in the United States.
On this same reasoning, many stores have an easy-open formula to open their tills (which silent alarms the police), or a harder version that implies calling a supervisor for everyday issues. Essentially, if you just open the cash register without a purchase, someone will know about it.
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.
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]."
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.
Military personnel on guard or patrol duty will typically have a "distress call-sign" which they use in place of their own to discreetly call for help.note For example, if his call sign was "Guard3", he would say over the radio "Dispatch, this is Guard7", which would alert the dispatcher that he is in trouble but can't say so. Another common tactic is to have a very specific but seemingly innocuous statement (Such as "Is Frank still dating Lisa?") which, when said, will alert their allies that something is wrong.
Arrested by the Nazis during World War II 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:
"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".)
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.
Other transponder distress codes include 7600, which translates roughly as "My radio is not working properly"note The transponder functions separately from the voice-based radio, so it's possible for one to be busted and the other to be working fine., so the Air Traffic Controllers will know why you don't respond to them on the radio, and 7700, which is a general-purpose Distress Call.
In World War II, when the Japanese let captured American soldiers speak on the radio to assure the troops they were being well-treated, they often said things such as they were allowed to keep their clothing — that is, they were still dressed in whatever rags their clothes were after battle and POW camp. This were improvised, but still conveyed enough to the Americans that they were not being treated well.
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 World War II 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.
Teens who frequently visit chat rooms and message boards normally only use general internet slang when they aren't being closely supervised by suspicious parents. When they are being closely supervised by suspicious parents, they use obscure or community-specific slang much more frequently, to let the other posters know that they're being watched.
Sometimes, more directly, they'll say something like "P.O.S." - "Parents Over Shoulder".
The Special Operations Executive (SOE), which sent many agents into occupied countries during World War II, provided them with duress codes and other security checks that would blend in with the normal textnote such as using non-obvious codewords and including/not including certain punctuation characters in certain points of a message. However, when the Gestapo forced captured "pianists" (wireless operators) to send false messages or attempted to use captured equipment themselves SOE headquarters would infamously ignore their own security protocol and keep trusting the communicationsnote one problem being that radio operators (hastily trained and transmitting under difficult circumstances) would often make mistakes anyway, making it harder to spot actual duress codes. On one occasion, the British contact in England told the agent attempting to sound the alarm that 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, with many agents being delivered directly into the hands of the Gestapo after the SOE had arranged drops with a captured agent or an enemy operator on the other end.