Imposter Forgot One Detail

Basically, someone has been body-snatched, copied or impersonated, but some mannerism, trick of speech or other attribute has been lost in the snatching/copying. Subtrope of Spot the Impostor.

Compare to Something Only They Would Say.

Expect spoilers!

Anime and Manga

 * In Magical Girl Lyrical Nanoha A's, when the masked men take on Nanoha and Fate’s identities, parts of their barrier jackets are not the correct color. This is likely because.
 * In Fullmetal Alchemist, Hughes is able to realize that Maria Ross is being impersonated by Envy because she doesn’t have her mole..
 * In Naruto, a Rain Ninja impersonating Naruto puts his shuriken holster on the wrong side (having a different dominant hand) and doesn’t include the cut Naruto got from Anko. Sasuke says this lack of attention to detail makes the Rain Ninja worse than Naruto.
 * In a filler arc, Naruto, Kiba and Hinata invoke this trope to distinguish themselves from their impostors. They have Hinata, who injured her leg, stay behind, and have the real Kiba be identified by having Akamaru with him, and the real Naruto identified by the chest.
 * In Bleach Szyael makes clones of Uryu, Renji, Dondochakka and Pesche. Uryu and Renji’s clones’ hair is slightly different and they have markings on their eyes. Dondochakka’s clones have no spots on their backs. Pesche’s clones wear trousers rather than a loincloth. Szyael claims that this is not carelessness as much as modifying them to fit his tastes, and while Renji claims he is stupid for doing this, Uryu notes that Renji didn’t notice until he pointed it out.

Live Action TV

 * The former Trope Namer (as Impostor Jamie Has No Accent) is in the Doctor Who episode "The Faceless Ones", when Jamie has been captured by the Chameleons—and Chameleon!Jamie does not have his Scottish burr.
 * In the episode "New Earth", Lady Cassandra (who speaks in a rather posh fashion) takes over Rose Tyler's body. Aware that she doesn't sound like Rose, she asks her assistant Chip how she should speak to the Doctor, to which he replies "Old Earth Cockney". Hilarity Ensues.
 * In "Tooth and Claw", the Doctor accidentally ends up in the Scottish moors. The moment he realises they're in Scotland he switches to a Scottish accent and pretends to be a Scottish physician named Dr. James McCrimmon so that Queen Victoria doesn't shoot him. Later at dinner he slips back into an English accent and Victoria calls him out.
 * The "Datalore" episode of Star Trek: The Next Generation made clear that Data doesn't use contractions, while Lore can. In an inversion, even though Data is the person Lore is copying, Data was the younger of the two, and so technically "lost" the ability to use contractions. (In a now-infamous mistake, the "real" Data used a contraction in the episode's final scene, and depending on who you talk to it either wasn't noticed or they didn't have time to do another take. This also launched a thousand, or at least half a dozen, wild mass guesses when it was noticed.)
 * In Red Dwarf the impostor Lister can play the guitar really well. The real Lister only thinks he can.
 * In the same episode, Imposter!Kryten calls Lister 'Dave', while the Real Kryten always calls him 'Mr. Lister'.
 * Played literally in MythBusters when Adam and Jamie tried to disguise themselves as each other - despite much effort, vocal cues and accents ended up being the weakest part of the ensembles. Imposter Jamie did indeed lack the accent.
 * Although Adam did better impersonating Jamie than Jamie did impersonating Adam, in part because Adam frequently mimics Jamie for amusement.
 * In the "body swap" storyline of Buffy the Vampire Slayer, Buffy had Faith's Bostonian accent on certain words. (No one really noticed in-universe.)
 * The Bionic Woman and Gemini Man once shared a script about a lookalike for the title character infiltrating the agency where she/he works despite being ignorant of the main character's superhuman abilities. They are both assassins, targeting the main character's superior. At the climax, the hero(ine) and the double are both claiming to be the real deal; the hero(ine) proves his/her identity by using their special abilities—one by bionic-jumping to the top of a tree, the other by turning invisible.

Film

 * In Megamind, Megamind blows his cover in one scene by pronouncing Metro City as "Me-trocity". He just barely avoids doing the same earlier in the film.
 * In The Sorcerers Apprentice, sorcerers powerful enough to do so have the ability to morph into whomever they wish. When Drake Stone, a less experienced Morganian, attempts to trick Balthazar into giving him the Grimhold while disguised as Dave, he accidentally lets his British accent slip, giving himself away.

Fan Fiction

 * In the Fresno, California-based Doctor Who Audio Dramas' continuity, the David Segal Doctor traveled with mute reporter Susie Jo Parker. Although mute, Susie Jo did speak. Usually when an impostor took her place and didn't research her very well.

Mythology

 * The traditional Japanese greeting "moshi-moshi" is a version of this. Shapeshifting Youkai wouldn't be able to pronounce it right, giving themselves away.

Web Comics

 * In The Dragon Doctors, Elizabeth, disguised as Goro, uses the bond of enmity between them to hide her true form completely, even from Sarin; it would have worked, except that while Sarin indeed couldn't sense her true form, he couldn't sense Goro's true (male) form, either.

Western Animation

 * In Jackie Chan Adventures, Jade is able to spot Paco as a magical clone because he pronounces her name correctly, unlike the real Paco, who always pronounced it "Yade."
 * In ''Kids Next Door", when Numbuh One is trapped in a Lotus Eater Machine, he realizes what's going on when he points out that Numbuh Four can't swim.
 * In My Little Pony Friendship Is Magic, it's the principle failing of, as "getting into character" isn't their strongest suit.

Real Life

 * Older Than Feudalism: Shibboleths originate from biblical times and have been used throughout history to detect impostors (or people on the genocide list).
 * For example: the words "branza" and "carpa" were often used to distinguish native Romanian speakers from native Hungarian speakers. In both words the first 'a' (written with a circumflex) is pronounced as a close central unrounded vowel, while the second one is a schwa. Neither of these vowels exist in Hungarian, and many Hungarian speakers tend to confuse them.