It's a Costume Party, I Swear

A favorite trick of the Alpha Bitch is to tell the Ordinary High School Student that the High School Dance will be a costume party and that everyone has to dress up as, say, a pirate. She is, of course, lying, but the heroine believes her due to Genre Blindness. Therefore, the heroine shows up at the dance dressed as a pirate (or, more likely, something impossibly elaborate and completely bizarre, such as a pirate ship) and thus looks like a complete dweeb. Either Hilarity Ensues or the heroine discovers how to take this rather cruel Practical Joke and make the most of it, much to the Bitch's dismay. (In either case the heroine will be so Genre Blind, or so sweetly naive, that she fails to notice until she's at the center of the party and surrounded by people that she's the only one in costume.)

This can also be pulled by the heroine if it's directed at a character mean enough to "deserve it". However, an Unsympathetic Comedy Protagonist is allowed to pull it on someone not quite that mean.

This trope may result in An Aesop about trying to get In with the In Crowd. See also Underdressed for the Occasion, which can occasionally result from a similar prank. Not generally related to For Halloween I Am Going as Myself and tropes related to that.

Anime & Manga

 * In one episode of Revolutionary Girl Utena, Anthy was anonymously given a dress for an upcoming ball. It was from the resident Libby, Nanami, and was water soluble. Nanami then bribed one of the wait-staff to "accidentally" spill a bottle of champagne on her. Fortunately, Utena is there to save the day with aplomb...
 * A version of this occurs in Gankutsuou: Albert wears a pirate-like costume to the Count's after someone convinces him to, and yes, he looks ridiculous, even for a show made up entirely of Unmoving Plaid textures.
 * Happens twice to Tsukushi in Hana Yori Dango by the same group of girls.
 * In Nodame Cantabile, the S Orchestra plans to play a concert in costumes, but the concertmaster calls it off at the last minute. Not everyone gets the memo. Everyone shows up in formal attire except for the melodica soloist, wearing an animal mascot costume, and the (male) timpanist, who wears an enormous ball gown. Since the soloist stood off to the side and the timpanist was on a slightly raised platform, it kind of worked out.

Comics

 * Johnny Storm pulls this on Ben Grimm in an issue of Ultimate Fantastic Four. Despite the thorough humiliation of showing up at a fancy party dressed as Carmen Miranda when he's already a gigantic rock-skinned thing, Grimm hooks up with Alicia Masters shortly afterward.
 * Before getting over the prank and meeting Alicia, he reveals that he's tried and failed to kill himself due to his invulnerability and wonders if Reed can find a way to kill him. Then they try to time travel to undo his origin story. Johnny's prank tore open a still fresh wound.
 * Subverted in an issue of Archie Comics, of all things. Veronica attempts to trick Betty by telling her that the most formal dance of the school year has been changed to a country-style hoedown, and that she should dress accordingly. Betty believes her--but also passes the information on to her friends without mentioning the source, and since they regard Betty as trustworthy they decide it must be true. The news spreads to other students, the hired band, and the decorating committee, all of who change their plans and outfits accordingly -- so, on the night of the dance, it's the fancily dressed Veronica who ends up looking out place.
 * One of the first things Morpheus does in The Sandman after escaping from his confinement is to grab nourishment from a passing dream -- in this case, a dream where a man's gone to a glamorous party populated by celebrities dressed as a clown.
 * The second variety happens somewhat early on in WITCH. Two sisters from the same school as the main girls like to spread baseless rumours about other people, including the leads. They get their revenge, when Cornelia tells the sisters that an upcoming party will be a costume party (which it obviously isn't, because this example is here).
 * Occurs in this Dilbert strip.
 * A variation of this occurred in a Dennis the Menace UK strip in The Beano where Dennis convinced Walter it was dress up as Rolf Harris day at school. This was considered one of the former Beano editor's all time favourite strips and was reprinted in The History of the Beano.

Films -- Live Action

 * The Alpha Bitch character successfully pulled this on Amanda Bynes in What a Girl Wants. In that case, the heroine was told to dress informally for a very formal event.
 * It was subverted since she was initially mistaken for a model.
 * This was also done in Legally Blonde.
 * And done better in its stage musical adaptation.
 * This also appeared (somewhat) in Mean Girls, though (ironically given the plot of the film) it was more a genuine misunderstanding on the main character's part. She did not know that costume parties were just excuses for the popular girls to wear the skimpiest thing they can think of without being called anything derogatory, and she looks noticeably odd in a much more modest and traditionally scary Halloween costume.
 * Bridget Jones, protagonist of that movie, goes to a party that she thinks is a "Tarts and Vicars" party, as a Playboy Bunny. In this case, she's not alone at least. However, the other two ones who fell for it are men and thus disguised as priests.
 * This is actually similar to the Archie example above; in the book, at least, it's made clear that the party was a "Tarts and Vicars" party, but that the organizers (including Bridget's mother, who seems disinclined to pull this kind of prank on her daughter) changed their minds and failed to inform Bridget or her father.
 * Occurs to the king of France in the movie Start the Revolution Without Me. His wife (who isn't too crazy about him) originally told him that a reception was going to be a costume ball, then she changed her mind without telling him. Cue scene where the already socially awkward king arrives dressed in a ridiculous rooster costume, apologizing profusely to everyone within earshot.
 * Groundhog Day has a variation of this, with Phil's movie date showing up in a skimpy French Maid outfit and saying, "I thought we were going to a costume party!"; however, Phil joins in the gag himself by dressing up as Clint Eastwood's "Man with No Name" character.
 * Practically a Big Lipped Alligator Moment, since it's never explained who she is and why they're dressed up like that (which is as much a Shout-Out to Bronco Billy as it is to Eastwood's more famous role) just to go see "Heidi II".
 * There's actually a pretty obvious reason why he wants her in a French Maid uniform. Think about it.
 * Played with in The Final. The outcasts tell all of the popular kids that they are holding a costume party, and when they arrive, they get drugged and tortured as payback for years of abuse. The "costume" part was so that the outcasts could conceal their identities until it was time to strike.
 * In Mr. Hulot's Holiday, the vacation resort actually has a costume party, but the guests are so stuffy they all pass on wearing costumes, except for Hulot and a young lady.
 * One of the supporting characters in I Was a Teenage Werewolf is tricked by her devoted but prank-loving boyfriend into showing up at a party in costume.
 * A variant happens in Angus, Thongs and Perfect Snogging, the film adaptation of the Confessions of Georgia Nicolson series. Georgia attends a party dressed as a cocktail olive only to find that everyone else at the party is wearing much sexier costumes (CatGirls, fairy wings and so on), including her friends who were supposed to dress as food with her but changed their plans at the last minute. Georgia looks comically out of place and runs home in embarrassment. In the book, she mentions that she regrets dressing as an olive at a party, but doesn't go into detail about what happened.
 * The implication is that Georgia and her friends expected everyone else to dress sterotypically so they had planned to dress as food to be different. Georgia just missed the memo when the other girls changed their minds.
 * Inverted in Mystery Team. Jason shows up to a work party, stating that he needs his most convincing disguise yet... which apparently requires a sombrero, a mustache, a Pancho and a toilet plunger. The guard lets him in, where he walks past a sign reading "Costume Party."

Literature

 * In Neil Gaiman's Anansi Boys, Mr. Nancy (not the best dad) tricked his own son into going to school dressed as William Howard Taft on President's Day.
 * In Girl Talk, after Stacey The Great weaseled into getting exclusive hockey game tickets/exclusive party by feigning to be Katie's (newly rich, and dispising Stacey) friend to her Clueless Mom. The girls told her that the party was now a water/ice-themed costume party. Cue Stacey and her cohort showing up as a Mermaid and a Lobster, and getting laughed off the ice. An example of the good girls doing this, but unlike most examples, They were called out on it by everyone else.
 * Something similar happens in The Shelters of Stone, the fifth novel of the sprawling Earth's Children series. The main character, who has just arrived at the titular shelters with her fiancé, is given "appropriate" festival garb by the local Alpha Bitch. It turns out to be underwear in a masculine style, garnished with the ceremonial sash a teenaged boy wears to announce that he's ready for sexual initiation. (Of course, our heroine is a Mary Sue Classic, so guess whether she still looks good in it.)
 * Even though she's

Live Action TV
"Samantha: Oh, that -- that little double-crosser! Darrin: She said casual. Samantha: Well, she knew darn well I'd expect her to lie!"
 * Dick pulled a version of this on Strudwick in the final episode of 3rd Rock from the Sun. In this case, it actually was a costume party, but Dick told him that it was historically themed when it was actually sci-fi.
 * Dennis pulls this on Maya in an episode of Just Shoot Me. He gives her a memo about dressing up for Halloween, and she comes to work dressed as a fairy princess. After she figures it out, Dennis hands her another memo for "Topless Thursday".
 * A variation appears in Only Fools and Horses where Del Boy and Rodney are going to a fancy dress party as Batman and Robin. En route, they stop a mugging just by looking like the pair. When they get there they discover the host died the day before and the party has been cancelled... so they have enthusiastically barged in on his wake spraying silly-string over the mourners. Boycie, who met them at the door, could've mentioned this but didn't. It's later commented that the dead guy would have probably found it hilarious.
 * Trigger also didn't hear about the host dying, and also came in costume. He felt a right prat dressed as a chauffeur.
 * In Ugly Betty, Betty was told they were having a costume contest at work. When she showed up dressed as a butterfly and no one else was in costume, her response was, "Well then, I guess this means I win!"
 * In the pilot of Bewitched, the snobby Rich Bitch Sheila Summers pulled this on Samantha Stephens, telling her to dress casually for a formal party. This was subverted in another episode four years later when Sheila invited the Stephenses to another party, telling them again it was casual. Samantha dressed formally only to find out that it actually was casual.


 * A subversion happened on Sister Sister. Ray and Lisa were invited to a Halloween Ball on a boat. They both assumed it was a costume ball. Lisa came as a princess and Ray... as a bunny. When they arrived, it turns out it it wasn't a costume ball. Lisa is able to blend in by removing her princess crown, but Ray is humiliated. He tries to leave (but he can't because the boat is already out to sea), and eventually gives up, gives himself a fake name and makes fun of himself, becoming the life of the party.
 * Mentioned in Scrubs, where Dr. Cox tricked Elliot into dressing up. Confronted with it, he responds: "But I did dress up. I came as 'Someone who doesn't make a fool out of himself'", a.k.a. normal clothes.
 * This Noodle Incident event was apparently a formal party, and Dr. Cox's outfit for the aforementioned costume was an Armani tux, so he must've sure looked the part.
 * An episode of CSI had an Alpha Bitch victim doing a variation of this to a Creepy Child suspect in a flashback (said child trying to divert suspicion from her brother for the murder). She gave her a dress which had various comments written in UV ink, that didn't appear until she stepped into room the party was held in.
 * An inadvertent variant happened in the US version of The Office, where Michael thought that Diwali was like Halloween and told his girlfriend there was costume party. Both went in costume but she went as a cheerleader, while he had a second head he could remove when he realized his mistake.
 * She has nobody to blame but herself, but something similar happens to Pam in a later episode. While she is going to art school in New York, she works at a nearby Dunder Mifflin branch. On Halloween, she assumes everyone will dress up since that's what they do at the Scranton branch so she dresses up as Charlie Chaplin. When she realizes her mistake, she tries to reduce her humiliation by at least removing her hat, but that makes her look like Hitler.
 * Not really about a costume, but a space suit: In the Firefly episode "Bushwhacked", most of the crew are exploring/looting a wrecked spaceship while Simon and Jayne stay behind on Serenity. Initially, the group goes in wearing space suits, but they learn that there is oxygen on the ship and thus no need to keep the suits on. When the crew finds something besides booty on the wreck, Mal radios Jayne to join them in the downed vessel. Jayne tells Simon to come with him to the raided ship, adding to bring a suit. It takes awhile for Simon to actually get there due to difficulty finding his sister River, and when he finally does get there, Mal says, "Hi... What are you doing here and what's with the suit?", at which point Jayne bursts out laughing.
 * Happened in the shortlived Australian Sitcom Trial By Marriage, where the machinations of one character caused the three main characters to turn up to a formal garden party dressed as two priests and a tart.
 * Get Smart: As part of a plan to discredit Smart as a court witness, KAOS tips him off to trouble at an embassy reception, telling him it's a costume party - he barges into a formal affair in a chicken costume.
 * Inverted twice on Cheers. The first time was when Norm organized an office toga party and the second was when Rebecca asked everyone to dress in 19th century attire for the bar's centennial. In both cases, the organizer is the only one who dresses up.
 * The Australian series Lockie Leonard features Lockie being told by his girlfriend Vicky's mother than she'll be wearing a fancy dress at her birthday party, "a very fancy dress." He misinterprets this as you'd imagine and he shows up to a fairly sophisticated dinner party dressed as a loaf of bread.
 * In the Doctor Who special, "Voyage of the Damned," the rich snobs on the cruiseliner Titanic tell middle-class couple Morvin and Foon (who won their tickets in a contest) that the formal-attire dinner was fancy dress. The two end up sitting at a table dressed in ridiculous, purple polyester cowboy/girl outfits while the snobs laugh it up at their expense. The Doctor gets revenge on their behalf by sonic-ing a bottle of champagne, spraying the snobs.
 * In a Modern Family episode, Mitchell is told by his new co-workers that dressing up for Halloween is an office tradition. He arrives at in work in a Spider-Man costume only to discover no one else is in costume. (He later learns that the two he spoke to are the only two who ever dress up.) Hilarity Ensues as he puts a suit on over the costume and then spends the rest of the day trying to get an opportunity to change out of the costume.
 * In an episode of Welcome Back, Kotter, Epstein apparently pulled this on Horshack in their younger years (told via story in "The Deprogramming of Arnold Horshack"); he said that the school dance was a costume party, and Horshack came dressed like a baked potato (wrapped in foil).
 * Discussed and averted in an episode of As Time Goes By where Jean and Lionel suspect Sandy's boyfriend Harry of arranging such a scheme in order to get Sandy to dress up as a St. Trinian's Girl. They're all very relieved when Harry shows up in costume to pick Sandy up.

Radio

 * Once on The Jack Benny Program, Phil Harris found out Dennis Day could do a dead on impression of Ronald Colman, Jack's next door neighbor. He talked Dennis into calling Jack (as Ronald Colman) and asking Jack to come to a costume party at his house. The next night, the Colmans are getting ready to go to bed, when they hear a knock at the door. They answer, and Jack bursts in dressed as a cowboy. The Colmans are completely bewildered.

Theater

 * In Wicked, Galinda gives Elphaba her the black pointy hat to wear to the dance party, telling her it makes her look "sharp". When Elphaba shows up to the party, everyone goes silent and stares. Her response? Start DANCIN'!!

Webcomics

 * In this Saturday Morning Breakfast Cereal strip, a transparent practical joke causes a man to show up to a funeral in a gorilla suit.

Web Original

 * Harry Partridge's 50 Llamas Day, is sort of an example involving wacky office theme dress up days.
 * Cracked's #6 Real Old-Timey Photograph That Will Give You Nightmares

Western Animation

 * A partially subverted version is played out in Danny Phantom when Danny finally gets invited to a popular party at Dash's house. He is told that everyone has to wear expensive sweats. After selling a bunch of his dad's old junk to earn cash to buy the clothes, he turns up only to find that the actual dress code is "geek chic,". So everybody is actually dressed like Danny and his "loser" friends, not because Dash wanted to make fun of him, but because Dash didn't have the money to buy his own expensive sweats--having spent his last dollars on, ironically, Danny's garage sale.
 * It's open to debate if Dash wanted to make fun of Danny or not. Dash's change of plans was emailed to the other invitees and he claimed to have forgotten Danny wasn't in the group of people who received the email.
 * A The Weekenders plot involves Tino and Tish being invited to an alleged costume party. They mention nothing to Lor or Carver, thinking their feelings will be hurt because they weren't invited. At the end of the episode, Lor and Carver (who were invited, actually, and knew that the party wasn't a costume party) tell Tino and Tish that they could have cleared up matters if they'd known Tino and Tish had been invited (never mind that they never mentioned their own invitations.)
 * They never asked.
 * In an episode of 6teen, Caitlin and her date went to a normal clothes party dressed up in costumes and Caitlin was humiliated (Her date's costume wasn't nearly as elaborate). No-one told them it would be a costume party. They just assumed it would be because it was Halloween.
 * In Sabrina the Animated Series Gem invites Sabrina to a ball at her house and tells her the theme is country and western. Sabrina casts a spell to give herself an authentic costume and shows up to find out it was a formal ball. Perhaps in a bit of irony, before she casts the spell, she pulls out a red evening gown wondering if it's appropriate for a country and western party. If she had worn that outfit, Gem's plan might have backfired.
 * Garfield and Friends episode "Top Ten" featured Garfield announcing some Top Ten lists. One of them was a list of ten reasons Jon's dates ended in a bad way. One of the items in the list was Jon forgetting to check which kind of party he's invited to. As an example, the viewers were treated to a clip of Jon arriving at a party wearing a chicken costume while the others are normally dressed. Jon started arguing about what the invitation stated.

Real Life

 * This happened to actor/comedian Alan Davies one year at the British Comedy Awards. The invite claimed that the event was to feature a "Latin theme" and encouraged the creative industry types to show their flavour. Cue Alan dressing up as a matador, only to discover he was the only person in costume. On live television. He talks about the experience here.
 * A variation happened to Jenette Goldstein at her audition for the film Aliens. She assumed the title referred to illegal immigrants and went to the audition dressed as a slutty Mexican border crosser, only to find the other prospective actors wearing military fatigues. A reference to this ended up in the film when Goldstein's character Vasquez is joked about making the same mistake, and replies "Fuck you, man."
 * Due to trans-Atlantic differences in English, a "fancy dress party" can mean very different things depending on where you're from. An American would probably interpret it as a formal event, while someone from the UK would assume it's a costume party. Either way, Hilarity Ensues. Solutions: dress as James Bond or as Mr. Pink, or wear a princess dress.
 * Schools with uniforms sometimes have mufti days and there are always a couple of people who forget and turn up normally.
 * On rarer occasions, somebody may claim that the next day is a non-uniform day for the school and wait for somebody to turn up in non uniform. If they didn't check with anybody else or the school beforehand, it does border on ridiculously stupid if they actually go through with it.
 * When Ernest Shackleton's 1914 Trans-Antarctic Expedition put in at the South Georgia whaling station before beginning the expedition proper, the officers were invited to dinner with the governor of the colony. Hubert Hudson (navigator) was told by Frank Hurley (photographer) that it was a fancy-dress occasion, and was informed that the dinner started one hour later than it actually did, ensuring that he showed up dressed as Buddha (wrapped in a sheet, with the top of a teapot strapped to his head) in the middle of the dinner, thus earning himself the nickname "Buddha" for the duration of the expedition.