Mistaken for An Impostor

Yesterday, Alice was pranked by Bob, who came to work dressed as Superdude. Today, the real Superdude shows up unannounced. Alice sees Superdude and assumes it's Bob again. She says, "Ha, Ha, nice try Bob!" and tries to pull off his mask.

Basically whenever a celebrity, Superhero, Villain, alien, monster, etc. appears, but other characters who see it are not unusually excited, surprised, or frightened, because they automatically assume it's a hoax, on the basis that another character tried to do something like that earlier.

Related Tropes:
 * When Played for Drama, it can become Not a Mask. However, in modern usage, it's predominantly a Comedy Tropes, as seen below.
 * When it's Played for Laughs, this can become For Halloween I Am Going As Myself or, more often, Your Costume Needs Work.
 * Often a karmic punishment for someone who runs a Monster Protection Racket or was Crying Wolf.
 * The inversion of Mistaken for an Impostor, where a seemingly powerful being is actually a relatively harmless con artist or hoaxster in disguise, is a Scooby Doo Hoax.
 * If the audience knows that the real monster really was there, but the characters never find out, that's Not So Imaginary Friend or The Masquerade.
 * If they find out but only after it left and so they are unable to confirm it, it is Real After All.
 * If it's merely played with by having neither the audience nor the characters really know what really happened, see Maybe Magic Maybe Mundane.
 * It can be used as a misdirection technique by The Men in Black, the Chessmaster, the Devil in Plain Sight or a Clark Kenting hero.
 * It can also be used with an Identical Stranger plot, where the real star looks exactly like the prankster.

Anime and Manga

 * In Sailor Moon, the Three Lights are doing a movie that has a monster in it that looks a lot like the actual monster that the Senshi end up having to fight.
 * This may count as an inversion, though, when Seiya disguises himself as the monster, the actual monster scares the Senshi first. Also, Taiki and Yaten are playing along and saying "There's a monster! There's a monster!" and then the real monster comes. They run away doing fake screams and then comment that Seiya's getting into it too much. Taiki notes that Seiya had a chainsaw (part of the movie monster costume) but the real monster doesn't have one. Then Seiya comes along and they tell him that's enough scaring people, but Seiya is confused because he hasn't done anything yet and he asks where Usagi and her friends are. Then Taiki and Yaten realize that the monster was real and run off, leaving Seiya all confused.
 * An Inuyasha episode had a variant of this, the youkai had been dealing with several of Shippo's gimmicks and illusions, and when Inuyasha walked out of the tall grass, he thought it was another trick (and even the audience wasn't sure).
 * It wasn't a trick. As the youkai found out the hard way.
 * In an episode of the 2003 anime version of Fullmetal Alchemist, the Elric Brothers are shunned in one town they visit...for claiming to be the Elric Brothers.
 * It is implied that at least one townsperson knew the other set was fake, but went along with the ruse out of sympathy for the poor orphans.
 * Hayate the Combat Butler dealt with this (~Ch 255). The characters decided to have a costume party, and then a bunch of real monsters got summoned in their midst. Some of the characters were truly scared (and protected by the stronger characters), others recruited the monsters' services for their own tasks. Not all the characters seem to have caught on they were dealing with actual monsters.

Comics

 * Phoney Bone in the Bone series ran a dragon protection racket so that he could run off with all the town's valuables. The hoax becomes far more complicated when a real dragon, who happens to be a good friend of his cousin, intentionally gets caught in his fake snare just to see what he would do.
 * A variant in this Donald Duck comic, "Lost Valley": Donald, who through a series of unfortunate events was roped into becoming a tour guide in the Amazon, comes upon an evil intelligent ape and thinks it's Daisy, who was dressed up earlier as one of them to infiltrate their temple. He then tries to help "Daisy" out of her "costume".
 * Calvin is derailed by this while using his Stupendous Man costume to mess with his class and disrupt one of his tests. Nobody believes Stupendous Man is anyone other than Calvin in a hood and cape.
 * In one early story, Spider-Man was too ill to fight effectively and was easily beaten and unmasked by Doc Ock. His poor performance made everyone think that he was just Peter Parker pulling a really stupid stunt.
 * The Silver Age story "The Batman Nobody Knows" features Bruce Wayne camping with some scouts, and listening to them regale each other with their theories about who/what Batman really is. These stories range from plausible-but-inaccurate to the supernatural. Finally, Bruce leaps from the bushes in his Batsuit to surprise the campers. Much to Batman's amusement, the kids don't buy it.
 * There's an Archie Comics story in which Veronica has Archie masquerade as her dad at the school's father-daughter dance, since Mr. Lodge had to go on a business trip. She gets angry with Archie at some point and then he goes outside, where he finds Mr. Lodge, who cancelled said business trip so he could go to the dance. Mr. Lodge arrives at the dance, but Veronica, still angry with Archie, attempts to expose him and finds out the hard way that she's yelling at her father.

Film

 * In the So Bad Its Good comedy The Creature From The Haunted Sea, the main characters try to kill a Cuban guard and blame a sea monster. Guess what shows up...
 * Early in Austin Powers: International Man of Mystery, Austin knocks out a waitress and pulls off a wig, revealing that "she" is actually a man (and one of Dr. Evil's henchmen). Later, when he meets Basil Exposition's mother, Austin knocks her out thinking that she, too, is a man in disguise, but it turns out that she is not an impostor and just looks "mannish".
 * A similar thing to the above happened in the first Crocodile Dundee movie. On his first night in America, Dundee is in a pub and meets a 'girl' who turns out to be a transvestite. He cups the man's scrotum to verify this. Later at a party, Dundee meets someone he believes to be a man in drag, tries to find out the truth in the same way, and learns that it is, in fact, a woman. The woman does not seem to mind, and even looks very happy about it.
 * In A Bugs Life,
 * In The Brothers Grimm, the titular brothers run a monster protection racket, and are called in to clear out another monster infestation... that isn't one of theirs. Their employer, of course, thinks it's their fault and plans to trap them.
 * In Tombstone, Wyatt Earp enters a bar on his first arrival in town and introduces himself to the saloonkeeper...who seriously doubts that he is speaking with the famous Wyatt Earp.
 * In the first Scary Movie, Buffy (who had inexplicably turned into an Alpha Bitch by that point) is killed while thinking the killer is Cindy disguised as him. Even after the killer beheads her, she still keeps ranting about the bad taste of the joke, much to his frustration.
 * In Zorro The Gay Blade, this trope is batted about a few times when the Spanish governor of Mexico hosts a masquerade ball in an effort to trap Zorro. The Alcalde is thrilled when Zorro appears and triumphantly rips off the hero's mask to find his best friend, Don Diego de le Vega (George Hamilton). Other guests arrive, all dressed as Zorro.  Then when Zorro actually does appear...
 * In Spider-Man 2, due to Power Incontinence the web-head must use the elevator to leave the building he's on top of. When someone else gets on the elevator, he just thinks Spider-Man is just a guy in a spidey suit. To his credit he thinks it's a cool costume.
 * In Days Of Thunder after Cole Trickle wins his first race, his crew hires a stripper/prostitute to dress up as a state trooper and pull their motorcade over, where the woman fondles him, then takes off her clothes and presents herself to him. Later when Cole is in an accident and comes to in the hospital he assumes the beautiful doctor (then-unknown Nicole Kidman) examining him is a similar setup.
 * In The American President the titular character calls a lobbyist to see about arranging a date, only to be mistaken for a colleague who had wanted to give his Presidential impression. Said lobbyist has an Oh Crap moment when the President asks her to call the White House switchboard.

Jokes
"Q: What did Tarzan say when he saw a herd of elephants in the distance?
 * Played with in a string of elephant jokes.

A: "Look, a herd of elephants in the distance!"

Q: What did Tarzan say when he saw a herd of elephants with sunglasses in the distance?

A: Nothing. He didn't recognize them.

Q: What did Tarzan say when he saw a herd of giraffes with sunglasses in the distance?

A: "Ha! You won't fool me with those disguises this time!""

Literature
"Before I could fully recover, however, two or three of these admirers ran up to me radiating indignation, and told me that a public insult had been put upon me in the next room. I inquired its nature. It seemed that an impertinent fellow had dressed himself up as a preposterous parody of myself."
 * In the Judge Dee novel The Chinese Gold Murders, one character is a Master of Disguise . At the end, the Judge thanks him for scaring off enemies by pretending to be a ghost, and he tells the Judge he wasn't there and has no idea what the Judge is talking about.
 * In GK Chesterton's The Man Who Was Thursday, an actor named Wilks decided to parody "the celebrated Professor de Worms". He was too good and completely succeeded in convincing all and sundry that he was the genuine article.


 * Wyrd Sisters plays with this: winds up filling in for the actor playing him in a play. People normally don't see him because of the Weirdness Censor, but now they're expecting to see Death... and he gets stage fright.
 * The Shirley Jackson short story Louisa, Please Come Home concerns a nineteen-year-old girl who runs away from home and returns three years later only to find that she Can't Go Home Again. Because her family thinks she's an imposter after the reward money. Dramatic Irony ensues.
 * A disloyal subordinate uses this against Tuon after Tuon's accidental abduction in The Wheel of Time - Suroth spreads rumors among the troops that some traitor is out in the countryside impersonating Tuon, and should be killed on sight.
 * The Baroque Cycle - Jack Shaftoe, a.k.a. King of the Vagabonds a.k.a. l'Emmerdeur, in rags and a slave collar rides a warhorse into a Parisian fancy dress ball (escaping captivity and trying to brazen it out) only to be mistaken for the king (of France.) Until the actual king arrives in his l'Emmerdeur costume, at which point all the noble women who'd been giving their jewellery to the 'fake' l'Emmerdeur feel foolish, and life becomes more exciting and dangerous for Jack (yet again.)

Live Action TV

 * This trope was played deliberately on Big Brother Australia. The housemates had to play as paparazzi and were shown celebrity impersonators whom they had to photograph. The Pamela Anderson impersonator turned out to be the real Pamela, and the housemates were genuinely surprised by this.
 * In Ned's Declassified School Survival Guide, Cookie tries to get into the all-girls book club by dressing up as a girl, and Moze catches on. Then, when a teacher enters, Moze starts attacking the teacher, but finds out that the teacher really is a woman.
 * Supernatural features a writer who gets visions of Sam & Dean's adventures and uses them for plots in a series of books called "Supernatural." Sam & Dean get tricked into attending a convention for his fans, where almost everyone male is LARPing as them, and are forced to masquerade as just really obsessed fans in order to stop an actual ghost without interference. But later, two of the fans find out some of the secret and volunteer to help, despite the danger unwittingly telling Sam & Dean that it's what Sam & Dean would do. Then everyone who works at the hotel and all the fans witness the ghost attacking them, forcing them to accept the supernatural, but luckily for all involved, the author saves them.
 * The writer also being played by the executive producer of the show...
 * In a similar, though more comedic vein to Supernatural, The Ghost Busters does this at least a few times. The funniest is in "The Phantom of Vaudeville." The titular phantom mistakes the Ghost Busters as a comedy duo who'd stolen his act many years before--the kicker is that the duo actually existed, and had a third banana in the form of a man dressed as an ape . When the Phantom tries to forcibly unzip Tracy's "costume," he gets a...shall we say, less than pleased gorilla on his hands.
 * The Science Fiction Sketch from Monty Python's Flying Circus plays with this a bit: A detective thinks that a blancmange-shaped alien is the notorious criminal Jack Riley, a blancmange impersonator and cannibal whom he'd encountered before, coming to the police office to turn himself in. (It Makes Sense in Context...)
 * No it doesn't.
 * The Sarah Palin opening sketch on Saturday Night Live? Where Alec Baldwin mistakes Palin for Tina Fey?
 * In a Halloween episode of Hannah Montana, Miley, (as Hannah,) is mistaken for her evil identical cousin by her best friend, Lilly. The friend comments on how bad her imitation is.
 * At the end of an episode of Cheers, a patron comes into Cheers claiming to be Boston Red Sox star Wade Boggs. The gang assumes he's an impostor (don't exactly remember why), and bum rush the guy, dragging him outside and stealing his pants. When Carla rifles the guy's wallet, she finds his drivers license... which IDed him as "Wade Boggs". The gang decides this counts as a win, because they now have Wade Boggs' pants.
 * The Thin Blue Line: after a succession of student pranks for Rag Week, Inspector Fowler single-handedly arrests (and insults) a group of armed, masked bank robbers, assuming it's another joke.
 * An episode of Angel had the team going to a library, along with Lorne, a green, horned creature from another dimension. A librarian sees them, and after a moment of shock, decides that he's there for a children's storytime. She then comments that his horns need work, but must be difficult to put on. Lorne replies "You have no idea".
 * In the NCIS episode "Witch Hunt", the team bursts in on a suspect, guns drawn, only to find a Halloween party in progress. The partygoers clap; one of them says, "Great costumes, but you misspelled CSI."
 * Goes badly in Only Fools and Horses when Del hires a stripper to pretend to arrest his uncle for his birthday. After his car is seen driven erratically (by Rodney) Del assumes the policewoman who come to question him about the incident is revenge. He is arrested for indecent assault.
 * In an episode of The Drew Carey Show, Drew's brother faces termination for being a cross-dresser, just after being hired to work in the cosmetics department. Drew challenges fire-happy Mr. Wick to differentiate transvestites from actual women perusing the store. Wick believes he found a "bad" impersonator of Dionne Warwick, but it was actually her.
 * In an episode of Hermans Head, Louise answers the phone and laughs at the woman who says she's Maureen McCormick; AKA Marsha of The Brady Bunch. A minute or two later, Mr. Bracken storms out of his office and states that his good friend Maureen McCormick just called him and said that a woman with a voice like a cartoon character made fun of her. This doubles as a Lampshade on the fact that Louise is played by Yeardley Smith, who voices Lisa on The Simpsons.
 * On CSI New York, a badly-injured man who'd been buried alive staggers out of a cemetery and stumbles down the street, covered in blood and grave earth ... straight into a flash mob of people dressed as zombies. Nobody notices his genuine distress until it's too late, and he dies from his ordeal and previous injuries.

Radio

 * In one of the Sherlock Holmes shows with Basil Rathbone and Nigel Bruce, Watson, making his rendevous with Holmes, says that Holmes' disguise is too over-the-top to be convincing, because no woman is that ugly... The woman slaps him, and then Holmes shows up.

Theater

 * In Anything Goes, Bishop Henry T. Dobson boards the ship while Moonface Martin is also on-board masquerading as the ship's chaplain. The local authorities are warned that a criminal is masquerading as a priest and, predictably, haul off the protesting Bishop.
 * In Cash on Delivery, Eric Swan has coerced his friend and roommate Norman to dress up as a woman, whereupon the head of the Social Security, a female, shows up unexpectedly. Eric, thinking that the woman is a dressed-up Norman, proceeds to not onyl comment on how Norman "made the breasts too big" but proceeds to juggle and motorboat said mammaries.
 * A variation in Little Me: On a sinking ship, Noble Eggleston continually confronts the captain, who is trying to flee by disguising himself as a woman. Around the third time or so, he confronts the captain again, but this time it's actually a woman. He apologizes to her, saying, "I'm terribly sorry, madam. The captain wears the same kind of dress."
 * In "Nothing's On," the play-within-a-play in the comedy "Noises Off," a sheik comes to look at the house where a series of misunderstandings have led everyone to be very angry at the man of the house. Unfortunately, the sheik and the house owner are identical strangers (which the director sarcastically chalks up to the long-lost prequel). The sheik is promptly accused of trying to avoid "his" (the house owner's) lumps by donning a ridiculous disguise.

Video Games
"Guybrush: Sooo... you say you're human now, huh? Well, [snaps his finger] let's just see what happens when I ...PULL OFF YOUR MASK!! [jumps at LeChuck's face]
 * In the game Prototype, one of the abilities Alex Mercer gains is "Patsy", faking out the military by accusing one of their own of being a shapeshifting mutant. And, of course, since Alex himself is the shapeshifting mutant they're hunting...
 * In Suikoden V, Euram falls for one of these.
 * The main character from the second game has that happen to him when you recruit Hoi (a actual imposter of the hero) he gets actually beat up by a angry bunch of villagers along with the real imposter
 * In the online game Legends of Zork, at the end of the quest "Antharia Jack and the Hat Mislaid",.
 * In a sidequest in Paper Mario the Thousand Year Door, Mario is given the task of helping a die-hard Luigi fangirl meet the man himself. Unfortunately, Mario's brother is unavailable, so Mario has to dress up as Luigi and meet the fan instead. While he is doing so, the real Luigi shows up. Guess what happens.
 * The penguin detective also thinks that Mario, in is regular clothes, is Luigi, no matter who tries to correct him. When Bowser shows up and tries to intimidate the detective into handing over the Crystal Star, he is thus told that Luigi has already made off with it. He doesn't take it well.
 * Near the beginning of Super Paper Mario, Mario visits a wizard to get a new power, only for the wizard to assume he's an impostor because his appearance matches that mentioned in the Light Prognosticus prophecy.
 * In Tales of Monkey Island Chapter 2: The Siege of Spinner Cay, when Guybrush thinks the human LeChuck is an impostor:

LeChuck: Owwch!! Guybrush, stop that! [pushes him off]

Guybrush: [understands] Huh. Okay... maybe you are human. I still don't like you.

LeChuck: Be that as it may, we still must work together to retrieve this Summoning Artifact!"

Web Comics

 * Used to some extent in this Xkcd comic. Which could also be used to illustrate the trope.
 * In Yet Another Fantasy Gamer Comic this new Drow jester does a greayt Alzaer'bith! At least, she adapts, "drinks on me" being the nicest thing she heard in a long time.

Western Animation

 * In an episode of Strawberry Shortcake, Custard and Pupcake dress up as crows in hope of faking out a fairy who was Playing Sick. It doesn't work, but later, when a real crow shows up, the fairy derides it for trying to trick her again, and she even pulls out a feather before realizing it's the real thing.
 * Played with in the Phineas and Ferb episode "Get That Bigfoot Outta My Face". At one point, the titular characters rig up a false Sasquatch to scare their friends and Candace; when a much more realistic Bigfoot shows up, Candace runs it down talking about how clearly it's fake, then the kids run in terror when she is eaten by it.
 * There's an episode of Spongebob SquarePants where SpongeBob sees a gorilla and concludes that it must be Patrick in a costume. Patrick walks up, but the gorilla takes off its mask to reveal the real Patrick. The fake Patrick takes off its mask to reveal a gorilla. This is subverted later?when SpongeBob wonders why a gorilla is underwater. The Gorilla tries to explain why he's underwater only to say "They're onto us!" and ride off into the sunset on a pantomime horse.
 * Another SpongeBob SquarePants example: In the episode where Mrs. Puff is in jail and she likes it better than teaching at boating school, SpongeBob and Patrick tried to break her free. Of course, Mrs. Puff didn't want to leave. One of their attempts to get her out was donning perfect disguises of prision guards. After they left, two real guards came to see Mrs. Puff. Thinking that they were Spongebob and Patrick wearing disguises, she pulled off their faces.
 * On two occasions, SpongeBob is being kicked out of some place: the first time it was a tough-guy club and the second time it was a slumber party. Each of these times, someone who looks a lot like SpongeBob shows up. In the first instance, the bouncer tries to pull off SpongeBob's wig (the preceding scene suggested SpongeBob to get a new haircut) which looks like black greaser hair, only to see the real SpongeBob show up in a blatantly obvious rainbow wig, and let the Mistaken for An Imposter person inside. In the second, it is a girl who wants to go to Pearl's party, but is kicked out because the partygoers are annoyed with SpongeBob. She runs off crying, and SpongeBob comments, "Whoever that was, she was uuugly!"
 * An old Donald Duck cartoon, "Donald Duck and the Gorilla", had Donald's nephews pretend to be an escaped killer ape to scare Donald. He catches them, and when the actual killer ape shows, he slaps it around a few times before realizing what it is and running for his life.
 * Also, in another Donald Duck cartoon, "Lion Around", two of Donald's nephews dress up in a lion costume to scare Donald while the third nephew goes for a yummy pie. However, one slip-up has Donald discover who the "lion" really is and shoos the nephews out of the house. Then a real mountain lion shows up and goes to Donald's house in an attempt to eat him and the pie. However, Donald thinks the actual lion is just his nephews in costume and tries shooing it away, but the lion persists and enters his house. At once Donald becomes infuriated and even attempts to rip off the lion's head, but then one of his nephews knocks on the window and tries convincing Donald that the lion is real by showing him the costume, which the nephews had taken off. It takes Donald a few seconds to realize that the "lion costume" he attempted to "take off" is actually a real lion, whom he had just ticked off! Oh Crap!
 * In the Hey Arnold episode "Ghost Bride", this happens with two separate impostors before the real ghost shows up.
 * A good number of the Scooby Doo movies have a mundane impostor followed by a real supernatural entity.
 * An episode of Jimmy Two-Shoes, had Heloise and Beezy are fighting over who gets Jimmy as their houseguest. Lucius comes to Heloise's house when she has Jimmy to go over something work related with her. She mistakes him for Beezy in disguise, and launches him into the sky.
 * This is a case of Plot Induced Stupidity, since Heloise would usually be smart enough to realize that Beezy is too big to pose as Lucius. To make this point clearer, Beezy disguises as Heloise later in the episode, and Jimmy sees right through it.
 * The first episode of The Flintstones had Fred thinking Barney was throwing a party without him. In response he tells a friend to dress as a police officer and come to the party complaining about the noise. Said party turns out to be a suprise birthday part for Fred, who quickly gets into it. Naturally, the noise gets too loud, and a cop shows up...
 * In The Simpsons, Homer annoyed Moe one too many times, and Moe kicked him out of the bar. Soon Homer reappears, dressed in a top hat and natty suit, sporting a handlebar mustache, calling himself "Guy Incognito." Moe beats him up and kicks him out of the bar again (in an unconscious state), only to have the real Homer show up and remark on the uncanny resemblance. Link.
 * In Yogi's Great Escape, Yogi and his friends keep trying to scare away ranger Smith and a trapper accompanying him by dressing up as a ghost. Later on, when the ranger corners Yogi in a room, a real ghost shows up and Yogi assumes it's someone else in a costume.
 * An episode of Pinky and The Brain has the duo grow to gargantuan size, while Pinky wears a Gollyzilla costume. Brain hopes to let Pinky "rampage" a bit, and then come out to save the day. Unfortunately, Brain runs into the real Gollyzilla.
 * In an old Looney Tunes short, Porky enters a bullfighting competition and has two friends dressed as a bull for him to fight. The friends get drunk and wander off, so Porky ends up fighting the real bull. It's only when the two guys appear singing drunkenly on the sidelines that Porky realizes the trouble he's in and starts running.
 * It's done in Garfield's Halloween Adventure. The first time Garfield does that, it's just a kid in disguise. The second time... it's a scary monster disguised as Bedsheet Ghost. The third time... a monster wearing a mask that looks just like its face. The fourth time... a Bedsheet Ghost with, well, nothing under the "bedsheet".

Real Life

 * There is a disorder known as Capgras delusion, that causes the person affected to believe the people closest to them have been replaced by imposters.
 * Charlie Chaplin entered a Charlie Chaplin Impersonation contest.... and came in 3rd.
 * The contest was to impersonate the Tramp, one of Chaplin's most popular characters. The judges noted his perfect impression, but he lost points for showing up on the spur of the moment with no costume.
 * Larry the Cable Guy had a similar experience in Las Vegas at a club where people did celebrity impressions. He played himself, and interviews of people leaving said he did the jokes well, but didn't sound the same.
 * An audience member was asked to leave a showing of The Rocky Horror Picture Show because he was a Tim Curry impersonator. The member: Tim Curry himself.
 * When appearing at science fiction conventions, Claudia Christian of Babylon 5 fame will often recount the story of how she once attempted to join a B5 discussion group, only to be chased off by the regulars as an allegedly clumsy and unconvincing impersonation!
 * In the 60's, Brazilian rock'n'roll musician Raul Seixas once showed up to perform so high that he couldn't sing his own songs. That led to the audience thinking he was an impostor and calling the police.
 * Dolly Parton once lost a Dolly Parton look-a-like contest.
 * The late Jeremy Beadle hosted a hidden-camera prank show called Beadle's About during the 1990s. He would appear in disguise (usually as an authority figure like a policeman or traffic warden), wind up the victim a little more, and then reveal that it was all a set-up for the TV. The downside? There was at least one case of a member of the public mistaking a real policeman for Jeremy Beadle, and attempting to pull his "disguise" off ...
 * Similarly, Candid Camera host Alan Funt was once on a flight that was hijacked and flown to Cuba. Because everyone on the plane recognized him, he was the only passenger to realize that this wasn't a joke.
 * In 2008, Tina Fey became famous for playing Sarah Palin in a Saturday Night Live skit. When the real Sarah Palin later appeared on the show, she was briefly mistaken for Tina Fey.