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.

WARNING! There are unmarked Spoilers ahead. Beware.

Examples of Imposter Forgot One Detail include:

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 cats are colorblind.
In Fullmetal Alchemist, Hughes is able to realize that Maria Ross is being impersonated by Envy because she doesn’t have her mole. This doesn’t save him from being killed, nor does it save Ross from being framed for his murder in the manga.
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.
K-On! has a more benign case of impersonation, which is only spotted when the imposter uses the standard (and thus "wrong") Japanese Honorifics when addressing the other band members.

Comic Books

Prior to The Clone Saga, the Chameleon used synthetic androids disguised as his parents to discover Peter's relation to Spider-Man; these duplicates were almost perfect in appearance, personality, and almost memory, so much that Peter's Spider-Sense didn't go off until their suppressed programming did. The one missed detail occurred when they offered to take his Aunt May to Uncle Ben's grave on their anniversary. May had actually married Ben in a private ceremony a month before the formal one, and the only witnesses other than the priest were Richard and Mary Parker, Peter's parents; because the two androids didn't know May's anniversary - something that could not have been programmed into their memory by the Chameleon nor anyone else - May suspected something was wrong, and this eventually led to the ruse being discovered.
Anther "spot the clone" example, in The Simpsons, after Ned shows up after having gone missing for a few days, everyone notices that... he stinks. Of course, most people would stink after being lost in the woods for over a week, but Lisa points out that Ned had once held a church fundraiser ("Get Gritty with God") and had gone 40 days and 40 nights without bathing, and still smelled springtime fresh. Although, this is only the first Out of Character Alert that leads to Bart discovering he's a clone created by Kang and Kodos.

Fan Works

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.
The original Shadowchasers has this overlapping with Pop Cultural Osmosis Failure. Jinx is looking for the shapeshifting boggart in a wax museum (an incredibly dangerous situation, like looking for a needle in a haystack and the needle is trying to kill you). However, when she passes by a display representing the duel between Obi-Wan and Darth Vader in A New Hope, she notices that the Darth Vader statue has a green lightsaber (Vader should have a red one), and realizes that must be him.

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 Sorcerer's 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.
Wild Wild West; in an early scene, West realizes the man he's talking to is not President Ulysses S. Grant when the imposter (Artemis Gordon in disguise) uses a Harvard shot glass - the real Grant went to West Point.

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.

Oral Tradition, Folklore, Myths and Legends

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

Video Games

Bayonetta; the Joy who disguises herself as the heroine still has the halo, a clear indication that she is a fake.

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 Codename: Kids Next Door episode "U.T.O.P.I.A.", Numbuh One realizes he is trapped in a Lotus Eater Machine when he realizes that Numbuh Four is swimming with the others - several episodes have made clear Wally can't swim.
In My Little Pony: Friendship Is Magic, it's the principle failing of the Changelings, as "getting into character" isn't their strongest suit.
In an episode of Batman: The Animated Series, Clayface tries to fool a security guard by shifting form into the likeness of the guy's boss, but the guard is tipped off when he realizes Clayface doesn't have an accent. Then he remembers his boss is in Hawaii, and Clayface decides to skip the pleasantries and knock him out.
Camille Leon from Kim Possible. She's a shapeshifter that can do a surprisingly good impression of Ron - but as Kim realizes, Ron always has Rufus in his pocket. She has a hairless Sphinx in hers.
In one episode of Challenge of the GoBots, Cy-Kill, Cop-Tur, and Crasher try to pull the old Frame Up plot on the heroes, using illusions to disguise themselves as Leader-1, Turbo, and Scooter respectively and then wreak havoc. The real Guardians are able to prove their innocence later by showing footage of the Scooter imposter using blaster-weapons - the real Scooter does not have blasters.

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.
Anthony Gignac. A con artist who swindled investors of over $8 million in real estate scams. Part of his ruse was telling a mark that be was “Sultan Bin Khalid Al Saud”, a Saudi Arabian Prince, and his lavish lifestyle seemed to back up his claims of royalty; he was not above threatening to use his "father's" influence to scare detractors. (More than likely, King Salman bin Abdulaziz Al Saud never even heard of him.) A true Saudi Arabian prince is certainly of the Muslim faith, but Gignac drew suspicion when one intended mark saw him order prosciutto at a restaurant; pork being a food that is taboo in Islam. This led to more inconsistencies in his cover being noticed, leading to a full investigation, leading to his arrest and conviction. He is now (as of 2022) serving an 18-year sentence after pleading guilty.