The skill in question is not necessarily exclusive to the character, just rare enough to set them apart from the crowd.
The distinct characteristic of this trope is that there is no moral dilemma; the character is just acting carelessly. If, on the other hand, the character must risk drawing attention to themselves in order to achieve an important goal, like getting out of a dangerous situation, it's a Cover-Blowing Superpower. If the character in question is a doctor, it's Inconvenient Hippocratic Oath. If the skill is part of their fighting style, see Fighting Fingerprint. If the character is unaware of the revealed identity, see Amnesiac Resonance.
Bonus if the character is thought, by the characters or the audience, to be powerless or even dead. In this case, the revealing skill is a preamble for a "He's Back!" moment.
Compare and contrast I Never Said It Was Poison, where the character's identity is revealed when he inadvertently reveals he has specific knowledge only he would have access to.
See also Something Only They Would Say where a particular character trait gives the person's identity away instead of a unique skill. I Am Not Left-Handed is when their level of skill is being hidden due to an intentional handicap.
- Attack on Titan: Ymir casually reads the labels on some canned food, even though nobody else understands the language. This outs her as not being native to the Walls.
- Green Lantern: A running gag in Kyle Rainer's early career was that anyone who was familiar with Kyle's artwork instantly recognised the design ethos of the new Lantern's costume.
- Robin: Tim Drake learned the secret identities of Batman and Dick Grayson by watching news coverage of B&R's escapades, during which Robin performed a complicated gymnastics move — which it had been established could be performed only a handful of people around the world - out of whom the only one local to Gotham was orphaned circus artist Dick Grayson.
- In Death God of New York, Nick Fury's only experience with the paranormal seems to be that he knows Natasha can see ghosts. But once she learns to materialize spirit threads, she realizes she can't sense Fury's even after concentrating until she gets a Psychic Nosebleed, something that could only occur if he had training in suppressing his spirit energy.
- Androgyninja's A Drop of Poison: When Sakura helps some children in Wave with a funeral barge, Haku notes she's unusually familiar with Water Country funeral rites, in particular that she sings the funeral song confidently along with the children, rather than mumble or hum like someone unfamiliar with the song would.
- In In the Kingdom's Service, members of the Vale Secret Service or Atlas Special Forces are trained to fight with different weapons than their civilian identities to avoid being identified by their fighting style. For similar reasons, any flashy Semblances, particularly unusual ones, aren't to be used. Jaune slips up during a spar when he uses one of his opponent's daggers far more skillfully than his own sword and shield.
- In the Miraculous Ladybug fic Tandem, the class plays dodgeball, and suddenly, without even realising it, Marinette and Adrien switch to their practiced Back-to-Back Badasses routine, obliterating everyone else as a result. Luckily, they are the only ones to put two and two together... until Alya finds them suddenly being cozy with each other suspicious, and looks up footage from a recent battle.
- Batman: Under the Red Hood. After the Red Hood gets away from them, Batman and Nightwing review the camera footage.
Nightwing: I'm saying that our boy here has some skills. He's been trained, and trained well...like right there! (Batman snags Red Hood's leg with a cable as he's Roof Hopping, but he turns in mid-air and cuts the cable with his knife) That! He sliced that cable off his leg before it went taut. You don't just do that—that has to be practiced, learnt.
Batman: And then there's the knife...
Nightwing: What about it?
Batman: You know many knives that can cut my lines?
- Robin Hood (1973): Once tempted into the belly of the beast(s) by virtue of an archery contest with a kiss by his beloved as first prize, Robin Hood can't resist being a little too impressive for his own good, splitting a previously shot arrow with his own.
- In Crouching Tiger, Hidden Dragon, Shu Lien suspected that the governor's daughter Jen was more than she claimed to be, and had martial arts training. Shu Lien confirmed these suspicions by deliberately dropping a tea cup—Jen caught it in midair and returned it to the table, without spilling a single drop.
- Justice League. Bruce Wayne turns up in Barry Allen's secret hideout to recruit him for the League. Barry is giving some Blatant Lies that he's not a superhero (his heat and abrasion-resistant costume is for competitive ice skating) so Bruce just chucks a batarang at his face, fulfiling the dual purpose of making The Flash reveal his powers while revealing his own Secret Identity as Batman.
- In The Long Kiss Goodnight, the protagonist is a sweet innocent soccer mom with a memory loss - she hurt her head five years ago, and her entire life before that is forgotten. Shortly after the start of the movie, she has an auto accident, hits her head, and is suddenly creepily good with daggers. She says that she must have been a chef before the accident, but it's undeniable that she as well as her family have received the first clue that she was really an assassin.
- An Invoked Trope in the Burt Reynolds movie Malone (1987). Suspecting that Malone is a Professional Killer, the villains give a gun to the brother of a man he beat up and send him to confront Malone to see what will happen.
- Invoked Trope in The Man from U.N.C.L.E. (2015). Two thugs are sent to rob Gaby Teller and her 'fiance' Illya Kuryakin to see if he really is a harmless civilian. After they goad him for a bit, Illya beats one of them up. When called on this, Illya retorts that any Russian would have done the same, and it would be more suspicious if he didn't. They'd expect a KBG agent to kill the guy.
- In Pirates of the Caribbean: On Stranger Tides, Jack Sparrow recognizes Angelica due to a particular sword fighting technique she uses that's apparently unique to her.
- After RoboCop outperforms everyone at the firing range in RoboCop (1987), Lewis notices him twirl his Auto-9 before holstering it. Alex Murphy had been doing that in imitation of a TV hero his son watched.
- In Sherlock Holmes (2009), Sherlock knows that Professor Moriarty was the one who killed a potential witness, because he's the only man known to keep and use a spring-loaded gun in his sleeve and the dead man had powder burns on his face.
- In Sherlock Holmes: A Game of Shadows, Sherlock and Watson are able to deduce that the man hired for the hotel assassination was the legendary Cold Sniper Sebastian Moran, because shooting a man at night from at least six hundred yards away with a small-caliber bullet would have been impossible for any other Englishman, even with the help of a gun rest and a wind gauge.
- Terminator: Dark Fate: When someone expresses skepticism that she is a Super-Soldier from the future, Grace proceeds to cut a fly lengthways in mid-air with her knife.
- Two examples in The Stinger of The Wolverine. The metal stuff flying to the air announces, for those who realize things quickly, that Magneto is back and with his power restored. The people frozen in their tracks, a stunt from X2: X-Men United, announce that Xavier is alive again.
- The Executioner: While traveling with a truckload of drunken soldiers in Guatemala, an equally drunk Carl Lyons is handed a Galil assault rifle and invited to shoot a stray dog. Instead of snap aiming and jerking the trigger, Lyons flips up the tritium night sights, aims carefully and kills the dog with a single shot, then puts on the safety before handing back the weapon. The soldier's commander comments that Lyons is not the innocent American tourist he's claiming to be.
- In The Fifth Elephant, Vimes figures out another character was trained by the Ankh-Morpork Guild of Assassins when he throws a small orange at him and the character doesn't react. Most people would react instinctively and either duck or try to catch it, where an Assassin would have the skill and training to see it coming, recognise it wasn't a threat and ignore it rather than be distracted.
- The Fourth Protocol. A man being followed by MI-5 gives himself away as an (incompetent) intelligence agent when he runs across the road and checks to see who's following him. No-one is, because one of the MI-5 Watchers is already on his side of the street.
- Harry Potter:
- A variation in Harry Potter and the Order of the Phoenix; Sirius Black accidentally reveals his presence in London when he accompanies Harry to the train station at the beginning of the year. He just wanted to get out of the house and have fun, but unfortunately the Malfoys know that he is an animagus who turns into a big, black shaggy dog...
- In Harry Potter and the Deathly Hallows book, Harry is revealed when he uses the Disarming Charm — his signature move — against Stan Shunpike, an innocent victim being mind-controlled into aiding the Death Eaters. (Which is, of course, why he used the Disarming Charm rather than something more deadly.)
- In the Inheritance Cycle, Eragon recognizes a Rider he's fighting as Murtagh when he flourishes his sword in a certain way.
- In The Lord of the Rings, Frodo accidentally gives himself away at the Prancing Pony by using his ring to turn invisible.
- In Les Misérables, Valjean makes Javert suspicious when he exhibits a feat of strength similar to one he performed before he became a wanted fugitive.
- In the seventh volume of The Unexplored Summon://Blood-Sign, Aoi plays with some children in a river, during which she squirts some water with her hands. Kyousuke notes to himself that there are multiple ways of doing that, with different regions having different variations of the technique. The one "Aoi" used is the same one Kyousuke taught to the White Queen. However, the end of the volume also reveals that the White Queen was planning on losing to Kyousuke the entire time, suggesting that she may have done this on purpose to reveal her identity.
- In the pilot episode, Oliver Queen is having dinner with his family to celebrate his return. Their Russian maid stumbles while serving Oliver, who catches the bowl she was carrying and answers her apology in fluent Russian, to everyone's surprise. Oliver deflects the question of when he learned to speak the language, but it's our first hint that Oliver did not five years marooned on an island like everyone thinks.
- In the second episode Oliver saves the life of his bodyguard John Diggle by throwing an unbalanced kitchen knife from ten feet away, accurately enough to knock another knife from his attacker's hand. Oliver tries to pass this off as luck, but Diggle knows the skill required to pull that feat off and starts to suspect Oliver's Upper-Class Twit act is a façade.
- In "Darkness On The Edge Of Town", Oliver first realises that Malcolm Merlyn and the Dark Archer are the same person when he fires an arrow at the billionaire executive only for him to do an Arrow Catch.
- In "Starling City", someone pops a champagne cork which goes flying towards Oliver's head, but he instinctively catches it similar to how he'd do an Arrow Catch. The Villain of the Week, who happens to be at the same party, is then shown giving Oliver a Meaningful Look.
- In Covert Affairs, the bad guy of the week has Annie drive his high-end sports car. At first, she pretends not to know how to drive a stick and grinds his gears, causing a pained expression to appear on his face. Then as she's driving along a country road, he keeps insisting that she go faster. Just as she almost has him convinced that she's just a regular girl, another car drives onto the road, forcing her to veer to avoid it. The bad guy notes the skill with which she has avoided the collision and asks where she learned to drive like that.
- Daredevil (2015)
- In a flashback to when he was dating Elektra Natchios, Matt Murdock takes her to the gym where his father used to train. While they're standing in the boxing ring she takes a swing at him which he easily dodges, confirming her suspicions about his Obfuscating Disability.
- In Season 3, Wilson Fisk watches on CCTV as Matt Murdock dodges an assassination attempt and then fights his way out of a prison riot. His suspicions were first aroused in the previous season when Matt punched him—he puts the skill and force of the blow down to Matt being the son of a boxer, but after thinking about it Fisk calls for the files on Daredevil, having fought him before.
- In Kaizoku Sentai Gokaiger, Joe sees Barizorg perform a sword attack that his friend and mentor Sid Bammick had taught him years ago.
- The Lord of the Rings: The Rings of Power: Sauron's unmatched blacksmithing skills and power to influence others are what blow off his cover as the human Halbrand. In Eregion, when he tries to help lord Celebrimbor, the best Elven smith, Galadriel gets suspicious of Halbrand's blacksmithing, which exceeded the knowledge of humans, even Elves. Then Celebrimbor starts talking about seeking to craft the power of the Seen and Unseen World, which is something the orc Adar warned her that only Sauron sought to do.
- Person of Interest.
- An Invoked Trope in "Prisoner's Dilemma". Agent Donnelly puts John Reese in with the other prisoners at Rikers Island, hoping that his military skills will be revealed when the other prisoners attack him. Reese has to take a beating just to maintain his cover identity (fortunately a Friendly Enemy is on hand to interrupt before things get fatal).
- In Season 4, Reese has a fake identity as an NYPD detective. When called on to work as a tactical instructor, a woman working for Internal Affairs notices that he uses military tactics, despite not showing a military background in his file. Reese claims his first instructor was in the Gulf War, so he must have picked it up there.
- Stargirl: When the Mahkents come over to the Dugans for dinner, Courtney brings over a hot dish straight from the oven in mittens, having already burned herself. She's shocked when Jordan Mahkent casually picks up the dish with his bare hands and doesn't even flinch, as he's paying attention to something else at the moment and doesn't realize the dish is hot. Courtney realizes that he's the supervillain Icicle.
- Wiseguy. A woman Vinnie Terranova is dating realises he's an undercover fed because, among other clues, he remembered the license plate of their rental car. Police officers have good reason to memorize license plates, but not the professional criminal Vinnie is pretending to be.
- Robin Hood: In many versions, Robin's identity is revealed when he exhibits Improbable Aiming Skills at an archery contest.
- Classical Mythology: When Daedalus was in hiding, his identity was finally revealed when he solved the problem of passing a string through a sea shell (by tying the string to an ant's leg and making it go through the shell), showing his rare intellect.
- The Mythic Dawn sleeper agents in The Elder Scrolls IV: Oblivion don't hesitate to summon up their distinctive magic armour whenever a fight breaks out.
- In Final Fantasy XIV, during the second expansion Stormblood, famed Kugane mercenary Yojimbo summons Dragon Head adds against the player during his first appearance in Kugane Castle. While it is easy to write off as the game designers just reusing old mechanics for a new fight, it actually foreshadows Yojimbo being none other Gilgamesh, the other boss known for using this skill.
- Knights of the Old Republic: When the Handmaidens inform the Exile that they noticed Atton Rand drop automatically into an Echani fighting stance, this gives the Exile the first hint that Atton is not the fool he pretends to be.
- The Pokémon Zorua and Zoroark, both possessing an ability which disguises them as another of the trainer's Pokémon, can be recognized when they use moves that their disguise could not possibly learn. Optionally, you could just hit them. In addition to that, their Dark typing does not change, so their illusion can be revealed but not broken when this apparently not-Dark-type is left totally unaffected by a Psychic-type move.
- A variant in Ace Attorney Investigations: Miles Edgeworth. Cammy Meele is revealed to be a murderer due to their ability to speak and read Borginian. They are known for possessing this skill (indeed, her major job with the airline requires knowledge of it), but it comes into play when Borginian cloth is used to clean up a murder victim's blood. The crate of Borginian cloth was labeled in Borginian, while another box was clearly labeled "Bedsheets" in English - only someone who could read Borginian would go for the box of cloth instead of the box anyone could know the contents of, and Cammy Meele was the only suspect on the plane who knew Borginian.
- Daughter of the Lilies: Teased at when Lyra asks Thistle how she can be reading a book in a dark cave. It's because she's a cave elf and therefore has Innate Night Vision, but she avoids the question and the others don't make the connection.
- Girl Genius:
- Gilgamesh Wulfenbach suspects that Agatha Heterodyne is a Spark, so he puts her in a flying machine he hasn't got working yet and drops them out a trapdoor from a zeppelin. Faced with certain death, Agatha redesigns the entire engine on the spot. They nearly get killed twice, but hey... For Science!
- Sparks in general are noted to have a particular "style" to their work that's consistent among their creations, and eagle-eyed people like Klaus Wulfenbach can pick up on them. For example, the Other's technology has the same "underlying principles" as Lucrezia Mongfish's work, which leads Klaus to suspect she was the Other; but Sparky styles can run in families, which causes Klaus to suspect the Other may be her daughter Agatha Heterodyne.
- Subverted in one scene, where Gil is able to recognize a new slaver engine as being made by one Count Maganox ... because the Count signed it.
- Avatar: The Last Airbender: Aang inverts this with respect to Korra: while Korra usually reveals herself as the Avatar by bending more than one element, Aang most often blows his cover by reflexively airbending when he really shouldn't. As the last airbender, this immediately identifies him as the Avatar to both friends and foes. Of particular note is when he airbends in Fire Nation territory — they were the ones responsible for the genocide of his people in the first place, and they did it to get to him.
- In one episode, he uses his airbending skill so Katara can pretend to be an earthbender. "Conveniently", she just happens to do this in front of Fire Nation soldiers who promptly arrest her and take her to the prison for earthbenders. Which is on a metal platform. In the middle of the ocean. And since Katara's a waterbender...
- In "The Waterbending Master", Zhao sees Zuko's dual dao swords, the weapon of the Blue Spirit. Zuko tries to claim that the swords are just a decoration and he doesn't know how to use them, but Zhao makes the connection, and attempts to have Zuko assassinated because of it.
- In The Legend of Korra, the title character joins a pro-bending team, the Fire Ferrets, as their waterbender. During her first match though, she accidentally reveals herself to be the Avatar when she earthbends to block an opponent's attack.
- Kipo and the Age of Wonderbeasts: In "Mute-Eat-Mute World", this is combined with Face-Revealing Turn. When they're navigating the sewers, Jamack realizes something is up with Kipo when she is able to see just as well as he can even though it's much too dark for a human to see. Then she turns and reveals her Animal Eyes.
- Star Wars:
- Star Wars: The Clone Wars: After leaving the Jedi Order, Ahsoka Tano befriends the Martez sisters, who are from the lower levels of Coruscant where the Jedi aren't much seen as heroes. While she initially successfully conceals her Force powers, in "Deal No Deal", Rafa points out that Ahsoka is unusually well-informed for someone who has supposedly always been a civilian, though she deflects by claiming she just pays attention to the world around her.
- Star Wars Rebels: "Breaking Ranks" reveals the Empire takes note of cadets who perform exceptionally well, and has them tested for potential Force-sensitivity so that they can be made into Inquisitors or made to serve the Empire in other ways. Notably, this is testing for a Revealing Skill that the tested don't even know they have.