A Disguise Trope. The Secret Identity of a character is revealed when the character carelessly exhibits a rare skill that they are known to possess. 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. 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.
- In the backstory of the current Robin (Tim Drake), this is how he learned the secret identities of Batman and Robin (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 by orphaned circus artist Dick Grayson.
- Similarly, in "Under the Hood" and the animated film based on it, Batman: Under the Red Hood, the identity of the eponymous Red Hood is revealed to Batman when he performs a certain maneuver he'd previously seen performed by Jason Todd.
- A Running Gag in early Kyle Rayner Green Lantern was that pretty much anyone who was familiar with Kyle's artwork instantly recognised the design ethos of the new Lantern's costume.
- 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.
- 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.
- 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.
- 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 X-Men 2, announce that Xavier is alive again.
- 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 anamagus 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 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 Kaizoku Sentai Gokaiger, Joe sees Barizorg perform a sword attack that his friend and mentor Sid Bammick had taught him years ago.
- In most versions of Robin Hood, his identity is revealed when he exhibits Improbable Aiming Skills at an archery contest.
- As the myth goes, when Daedalus was in hiding, his identity was finally revealed when he solved the problem of passing a string through a sea shell, showing his rare intellect.
- 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.
- 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.
- 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.
- 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.
- Aang from Avatar: The Last Airbender 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 ship. In the middle of the ocean. And since Katara's a waterbender...