It's a Costume Party, I Swear!

"I'm thinking 'Do I want to go?' Will it be everybody standing there in jeans and t-shirts, and I'm standing there as Little Bo Peep?"

— Sean Lock

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 look 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. Compare Prank Date. Contrast Uninvited to the Party, where someone is so uncool they're not invited. Not generally related to For Halloween, I Am Going as Myself and tropes related to that.

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Examples:

Advertising

• An ad for salsa has Jean-Claude Van Damme enumerating the types of salsa, said types being consumed by someone whose appearance epitomizes the flavor: hot (sexy guy); mild (frumpy woman) and garlic (guy in a vampire costume, who says "I thought it was a costume party!")
• An ad for Doritos, features a guy at a party dressed like Dracula, saying he thought it was a costume party.
• Snickers had a spot where a guy dressed as a centaur is on his way to a fantasy party. He got his dates mixed up as he had arrived at his pals' football fantasy party.

Anime & Manga

• Happens twice to Tsukushi in Boys over Flowers by the same group of girls.
• 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.
• 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.

Comic Books

• 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.
• Disney Ducks Comic Universe: One Donald Duck comic has Gladstone Gander trick Donald into believing the ball he and Daisy will attend is a costume party, and that Daisy wants him to dress up like a gnome (in hopes that Daisy, when seeing Donald in costume, would be so ashamed of him that she would ditch him in favor of Gladstone). Unfortunately for him, Daisy finds out and to simultaneously get back at him and make Donald feel less ridiculous, she TOO shows up in a gnome costume.
• One of the first things Morpheus does in The Sandman (1989) 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.
• 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.
• The second variety happens somewhat early on in W.I.T.C.H.. 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).

Comic Strips

• A variation of this occurred in a Dennis the Menace (UK) strip in The Beano where Dennis convinced Walter it was dressed 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.
• Occurs in this Dilbert strip.

Film — Live-Action

• 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 (Cat Girls, 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 stereotypically so they had planned to dress as food to be different. Georgia just missed the memo when the other girls changed their minds.
• 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; her father arrived dressed as a vicar, another guest arrived in a more elaborate bishop costume, and the only other "tart" hides in a nearby hedge until she can make a quick getaway while no one is looking. It's made clear that the party was a "Tarts and Vicars" party, but that the organizers changed their minds and failed to inform Bridget or her father.
• 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.
• 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.
• 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.
• Legally Blonde: Elle is invited to a costume party by Vivian and comes dressed as a Playboy Bunny among casually dressed college kids. Elle manages to work it anyway, but it is also this scene which finally convinces Elle that Warner is never going to take her seriously, which gets her pissed and gets her motivated to succeed. She is next seen at the campus store buying a high-end computer for her schoolwork... still wearing the costume.
  Elle: Don't ask.
  Emmett: [smirking] Wasn't gonna.
• A variant in Let's Be Cops. Ryan invites Justin to a costume party, and they wear their fake police uniforms, only to find out everyone else is wearing formal attire with fancy masks. Ryan had never learned the difference between a "masquerade" and a "costume party".
• It happens to Chevy Chase in the 1995 movie Man of the House. Jonathan Taylor Thomas tells him to wear full Native American regalia to an Indian Guide barbecue. The barbecue is, of course, a casual event, and the other tribe dads have to tell Chevy that the Indian garb is better suited for formal tribe events.
• Played with in Mean Girls: (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.
• 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.
• 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 poncho, and a toilet plunger. The guard lets him in, where he walks past a sign reading "Costume Party."
• Barely averted in Oxford Blues. The mean boys tell the protagonist that the party is to be traditional Scottish dress — a suit with a kilt. He arrives duly outfitted, but just as he gets to the door, he sees that no one else is wearing a kilt, and leaves before anyone sees him.
• 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.
• In That Awkward Moment, Jason gets invited to a "dress-up" party by Ellie, showing up in a ripped T-shirt with "Rock Out" on the front and a large dildo hanging out of the front of his pants. It's fancy sort of party where everyone dresses up. He manages to pull it off by basically refusing to be embarrassed about it.
• 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 - in this case a fashion show. This backfires on Clarissa however when Daphne's casual outfit leads people to assume she's one of the models, and she's put on stage - where she steals the show.

Literature

• Accidental Detectives: Mike and Ricky are best friends but they do enjoy pranking each other, and the only prank Ricky holds a grudge for is Mike using this trope to get him to dress like a banana for the (formal) birthday part of the prettiest girl in their class.
• 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 one of the Captain Underpants books, George and Harold pull this on the entire school when they tamper with the weekly announcements and tell the kids to come to class picture day in bumblebee costumes. However, in this case, the prank is on the school, not the students.
• In Barbara Wersba's The Dream Watcher a dance called the "Tramp's Ball" was held at Albert's school and he assumed that it would be a costume party. He showed up dressed as a fairly convincing tramp, only to discover that all of the other boys were wearing shirts and old jeans, while the girls had on pantsuits.
• Something similar happens in The Shelters of Stone, the fifth novel of the sprawling Earth's Children series. Ayla, who has just arrived at the titular shelters with Jondalar, is given "appropriate" festival garb by the local Alpha Bitch, Marona, who wants to humiliate Ayla for "stealing" Jondalar from her. 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. However, the trick backfires as the clothes actually look good on Ayla, who later takes to wearing a modified version of the outfit while hunting.
• Played with in the Elephant & Piggie book I Am Invited to a Party!. Piggie and Gerald receive invitations to a party and Gerald, who claims to "know parties", suggests that they dress up for the party in case it's a fancy party. Then he says it might be a pool party too, so they throw on scuba gear over their fancy clothes. Then he says it might be a fancy pool costume party, so they put on costume masks along with everything else they're already wearing, despite Piggie's clear skepticism about his claim. When they finally arrive at the party, it turns out that Gerald was 100% right about it being a fancy pool costume party.
• In Girl Talk, after Stacey The Great weaseled into getting exclusive hockey game tickets/exclusive party by pretending to be Katie's (newly rich, and despises 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.
• One of the Horrible Histories books has a section on Lord Lucan, which discusses his attempts to gaslight his wife and get her certified as insane. A picture accompanying this section has Lady Lucan dressed in a chicken costume and shouting "But I was told it was fancy dress!" to a room of normally-dressed, bemused-looking partygoers, as Lord Lucan whispers to one of them "See what I mean? Nutty as a fruitcake."
• In Reserved for the Cat, one of the dreams that the troll gives minor character Terrence is of him being given an invitation that states "fancy dress", only to show up at the party to find everyone else is in evening dress.

Live-Action TV

• 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.
• 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.
• On Bewitched, Sheila (Darrin's ex) pulls this on Samantha (his wife) twice.
• On The Big Bang Theory, Stuart arrives at the guys' Game of Thrones viewing party as a member of the Night Watch, only to find that he's the only one in costume and it was just a prank by Howard.
• In the Castle episode "The Good, The Bad & The Baby", Castle tells Beckett that his family has a tradition to wear costumes for Thanksgiving. She shows up to dinner in a costume... but she's also suspicious enough about the whole thing to bring one for him as well.
• 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.
• 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 the room the party was held in.
• Doctor Who: In "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 sonicing a bottle of champagne, spraying the snobs.
• Father Brown: The Asshole Victim in "The Scales of Justice" throws a The Great Gatsby-themed party but tells his brother the theme is the Teddy Bear's Picnic. As a result, the brother arrives dressed in a teddy bear costume while everyone is in Roaring Twenties finery.
• Example with 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 ( evidence of Reaver activity), 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 a while 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.
• 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. The attempt backfires when a KAOS plant "identifies" the man who crashed the party, as the chicken costume had made him unrecognizable.
• Home Improvement:
  • Inverted once. Brad's girlfriend decides they're going to a Halloween party as Raggedy Ann and Andy. But then she gets mad at him over a minor matter...and shows up dressed as a biker, making his lame outfit look even lamer.
  • Tim invites a bunch of his friends over to work on his hot rod. When Al asks if there's a theme, Tim sarcastically says it's a costume party. Al then shows up dressed as a cowboy.
• Zigzagged on James At 15. On James' first day at his new school, he's the only one who's not wearing a suit. The other kids explain that he's expected to wear one the next day, and he does — only to find out that it's only on opening day when everyone wears suits.
• 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".
• The Australian series Lockie Leonard features Lockie being told by his girlfriend Vicky's mother that 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.
• While it wasn't an intentional prank, 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 almost 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.
• Night Court: A self inflicted version comes when Dan tries to sneak into the Distirct Attorneys Private Halloween party to social climb, dressing as Dorothy from ‘’The Wizard of Oz’’ to do so. When he arrives, everyone is formally dressed and he sticks out like a sore thumb.
• The Office (US):
  • An inadvertent variant happened in the US version where Michael thought that Diwali was like Halloween and told his girlfriend there was a 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.
• 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 the series 7 finale of Peep Show, Mark attends a New Year's Eve party dressed as a fisherman. The party actually is a costume party, but Mark feels out of place when several other men come to the party dressed as James Bond, meaning they get to wear tuxedoes while he's stuck in a large yellow mackintosh. Mark's costume becomes even more ridiculous when he visits several other New Year's Eve parties, none of them fancy dress.
• Reno 911!:
  • The show has a Running Gag where every year, Deputy Trudy Weigel shows up to work on Halloween in costume because every year, she's told that everyone's doing it.
  • In one episode, Dangle hosts a murder mystery dinner, telling the other deputies it's a "dress-up" party. Junior misinterprets this as you'd expect, and comes to the party in a Captain Jack Sparrow costume.
• Sabrina the Teenage Witch's Season 2 Halloween Episode involves Sabrina throwing a Halloween party, and Valerie turns up as a duck. Then when other kids arrive not wearing costumes, she's humiliated. She gets a bit of relief when real ghouls from the Other Realm show up, and she assumes they're guests in costumes.
• Scrubs:
  • Just mentioned, but 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.
  • A variation occurs in a flashback to J.D. and Turk's college days. Turk talks J.D. into going to a party at a black fraternity with J.D. in blackface and himself in whiteface, saying it'll be fine as long as J.D. stays with him. He then conveniently wanders off just as the frat members open the door, making it look like J.D. just showed up to the party in blackface.
  • Dr. Cox. did this to J.D. and Turk in Season 9. It didn't bother them at all.
• 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 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.
• Done at the end of an episode of Trapper John, M.D., Dr. Riverside and the other staff members are going to a (confirmed) costume party; he's waiting by the elevators, dressed as a Pilgrim — but everyone else is in their usual hospital attire, with no intention of donning costumes...leaving him looking awkwardly out of place, when they all get on to ride downstairs.
• Happened in the short-lived 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.
• 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!"
• Inverted in the Under the Umbrella Tree episode "Holly Gets the Blues." Holly accidentally falls face-first into a tub of blue fabric dye just hours before she's supposed to go to a party, and though she manages to wash the dye off her face in time, she can't get it out of her hair. But then she learns that the party is a costume party, so she goes after all, dressed as a blue-haired clown.
• 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).
• A variant in The Wild House, where Natalie's invitation to a party gets destroyed and she has to piece it back together. She misreads "old clothes" as "gold clothes", and shows up in a glamorous outfit to what turns out to be a paintball party.
• A variant happens in The Windsors, where there actually is a fancy dress charity ball being held, but Camilla manipulates Kate into going as a pirate with a peg leg. It turns out the party is for military amputees.
• The Worst Year of My Life, Again: In the April Fools Day episode, Simon pranks Alex into thinking it is 'casual clothes and weird hair day' at school, so he turns up out of uniform and with crazy hair. Thanks to the Loop Year, Alex turns the tables on Simon the second time around and he is the one who comes to school out of uniform and with weird hair.

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.

Theatre

• Done very well in the musical adaptation of Legally Blonde. Elle is called a skank by her romantic rival for her Playboy Bunny costume. She recovers by taking someone's glasses and insisting she is dressed as an undercover Gloria Steinem and asks her rival, loudly, if she is calling Gloria Steinem a skank. A horde of feminists descend upon the rival as the victim leaves.
• 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 at the party, everyone goes silent and stares. Her response? Start DANCIN'!!

Web Animation

• Harry Partridge's 50 Llamas Day, is sort of an example involving wacky office theme dress up days.
• In RWBY Chibi, Port once told Ozpin that a faculty party was a costume party as a prank. Ozpin later tries to get rid of the photographic evidence.

Webcomics

• Subverted in Homestuck during the Ministrife. Porrim thinks it's this, but Vriska keeps saying it's a fight while an entire ghost army collides.
• In this Saturday Morning Breakfast Cereal strip, a transparent practical joke causes a man to show up to a funeral in a gorilla suit.
• A non-malicious example in Whats Up Beanie: Beanie misinterpreted 'dress-up party' (as in a 'wearing fancy dresses' party) as a costume party and demands to be a pink cat. Beanie's mother makes it work anyway.

Web Original

• Cracked's #6 Real Old-Timey Photograph That Will Give You Nightmares
• The Nutters subverts this. A Wild Teen Party is held and Eoin is unsure of what to wear. Jake tells him to "dress up nice", so he dons an Elvis Presley costume. After receiving "that's disgusting! get that off you now!" in response, he appears at the party dressed normally.

Western Animation

• 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. It later turned out to actually have been a costume party, but everyone was too embarrassed to wear their costumes.
• A Halloween episode of The Casagrandes has Ronnie Anne and Sid attending a sixth-grader's Halloween party. The two girls show up in goofy costumes, but everyone else in that party is not, saying they're too cool for that.
• Harry accidentally does this to himself in a Cyberchase For Real segment. His friend gave him a coded note in a Viking rune cipher, and Harry translated the first part telling about the time, but only translated up to "Co" on the last bit and assumed that the rest meant "Costume Party" based on how long the words were. He arrived at his friend's house dressed in a homemade Viking outfit, only to have his friend correct him saying the note read "Concert Start".
• 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.
• Inverted in an episode of Doug. Doug is invited to a party at Beebe's house and ends up with a large pimple on his nose right before. The entire episode plays on his attempts to get rid of it and his embarrassment. When he finally sums up the courage to go, Skeeter informs him that is actually is a costume party, allowing him to wear a mask to cover it up.
• In the Duck Dodgers episode "That's Lifomatica" Dodgers invites his friends (and enemies) to a party to show off his Lifomatica system and Commander X-2 is shown in the middle of the party dressed as Little Bo Peep.
  Martian Queen: You know, it's not a costume party.
  Commander X-2: (testily) I know that now.
• 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.
• 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.
• 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.)

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.
• 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 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 ^note meaning casual dress 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.
• Inverted with Phineas and Ferb co-creator Dan Povenmire — he recounts how he was once invited to the birthday party of some friends' daughter but was not told that it was a Phineas and Ferb themed party. As such, he was the only adult not dressed as Dr. Doofenshmirtz — a character that he actually plays!
• According to this biography, J. R. R. Tolkien and C. S. Lewis once attended a non-costume party as polar bears, apparently For the Lulz.
• When The Nutters (mentioned above) was nominated for a Google Tech Festival award and the cast attended, Liam Gaynor was told that it was a formal event and arrived dressed in slacks, a shirt and nice shoes... only to find everyone else dressed casually.

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