Mistaken Identity

The ploy where one person is mistaken for another and Hilarity Ensues. Truth in Television to an alarming degree.

This was a big favorite of William Shakespeare, which means it has been riffed on by a large number of others. Shakespeare, of course, cribbed it from Greek plays, the Greek playwrights cribbed it from ... who knows? Pictures on a cave wall, maybe.

Subtropes:
 * Mistaken for Special Guest
 * Mistaken for Servant

Not to be confused with Thoroughly Mistaken Identity.

Anime & Manga

 * In Fairy Tail, Wendy is elated to be reunited with Jellal, who saved her as a child. Of course, since there's, like, ten Jellals running around, she's got the wrong one. It's actually a little sad, since the one she finds
 * A Running Gag in Yu-Gi-Oh! ARC-V is for a member of the main cast to be mistaken for another due to them being dimensional counterparts of each other with similar faces and hairstyles.

Comic Books

 * Flash Thompson has a tendency to dress up as Spider-Man, and end up mistaken for him by his enemies.
 * In Titans,  mistakes

Film

 * The German movie Kleine Haie is about some guys who want to be accepted at an actors' school. One of them in fact starts out as a worker who just had to deliver a chair to the school, gets mistaken for another wannabe actor, but the teachers mistake his behavior "Hey, I just want to deliver this chair!" for improvised acting. He gets the hang of it.
 * The premise of the film Top Hat is that Dale Tremont mistakes Jerry Travis for Horace Hardwick, her friend's husband. They met when she was disturbed by his loud dancing, they hit it off and the next day she asks the hotel staff who was in the room above, they give Mr Hardwick's name (due to an error Jerry had to share his room) and point out Jerry when he comes to return the keys. Throughout the rest of the film, Dale is struggling with being whooed with her friend's husband, so she thinks... it gets even funnier since Mrs Hardwick was planning on introducing Dale to Jerry in hopes they would hit it off.
 * A group of nihilists rough up The Dude. They do this because they're looking for one Jeff Lebowski, who The Dude has the same name as. A long stretch of strange events ensues.

Literature

 * Happens twice in The Adventures of Huckleberry Finn. The first time is intentional - two con men show up at the home of a recently deceased man, claiming to be his brothers who live in England. This fools the foolish townspeople for some time, until the real brothers show up. The second plays the trope much straighter: Huck just happens to unknowingly show up at the house of Tom Sawyer's aunt hundreds of miles from home, who just happens to be expecting Tom for a visit. Luckily, when Tom shows up, he plays along and poses as Sid Sawyer, Tom's own younger brother.
 * Well, younger cousin; Sid's mother is Tom's Aunt Polly - the one he's visiting is his Aunt Sally who has never seen either of her nephews. So when a preteen boy shows up at her house she automatically assumes it's the one she's expecting. Tom being Tom, he thinks this is a great joke/adventure, and turns it into a game in which he makes up all the rules.

Live-Action TV

 * An episode of House featured a sort of "torn from the headlines" mystery in which they spent the entire episode treating a woman who survived a building collapse but horribly maimed and unable to communicate. They keep finding inconsistency after inconsistency with her condition, medical records, and patient history (as provided by her fiance and mother) but assume that House's "everybody lies" motto explains things one way or another. In the end,

Newspaper Comics

 * This Garfield strip has Garfield mistaking an actual bear cub for his stuffed bear.

Radio
In one episode of The Shadow the villain believes that Margo Lane is The Shadow and that whenever they appear together she is simply using ventriloquism.

Theater

 * Shakespeare's plays:
 * The Merchant of Venice—Explores the differences between outer appearance and inner reality, putting almost every character in a disguise at one point or another.
 * As You Like It—Shakespeare had a lot of fun with utterly changing the meaning of the dialog by letting the audience in on who was really speaking, but leaving the characters unaware.
 * Twelfth Night—Malvolio, Orsino, and Olivia all find themselves doing things they wouldn't ordinarily, by trying to act as the person they are mistaken for.
 * Measure for Measure—The Duke uses disguise to reveal the truth about Angelo's character. This also provides comedy when Lucio speaks of the Duke to the Duke while unaware of the Duke's identity.
 * The Comedy of Errors—The entire play is built around the chaos two pairs of long-separated identical twins cause when they coincidentally end up in the same town.
 * Julius Caesar—Following Antony's effective speech, an Angry Mob forms up to find and kill Caesar's murderers. They run into a poet named Cinna, the same name as one of the conspirators, and beat him to death despite his loud and repeated protests that they have the wrong man.

Video Games

 * A funny meta example, fans of Yume Nikki stumble onto this Youtube channel, thinking it's the same Kikiyama who made this game.
 * Happens in Chrono Trigger near the start of the game when Marle goes to the past and is mistaken for her ancestor, Queen Leene.
 * In Battle Golfer Yui, Hagata thinks that Yui is his old rival Hoshi, while Kitako thinks that Yui is a Nurarihyon.

Web Comics

 * Occurs in this And That's How Equestria Was Made! strip, with Spike mistaking the sea serpent from The Elements Of Harmony for his crush, Rarity.
 * In Sinfest, Death is blamed for Michael's death. So he disguises himself -- and is taken for him.

Western Animation

 * In the Garfield and Friends episode Mistakes Will Happen, Garfield and Odie are watching the intentionally mistake-filled episode of Garfield And Friends itself, Mistake Will Happen. At one point in it, Garfield and Odie overhear Jon planning to scare them using a ghost costume as punishment for walking in the woods alone. However, an escaped convict sees him afer he bumps into a tree, and is worried that there might be people, so he uses the costume to scare away any people. Garfield and Odie spot him thinking he's Jon, and Hilarity Ensues.
 * Played for Laughs in Histeria!, where a Running Gag on the show would have Pepper Mills would repeatedly mistake every historic figure she came across as either a modern celebrity or fictional character, often one who was not even remotely similar. A complete list is found here.

Real Life

 * People often don't realize how huge a problem eyewitness mistaken identification is. If there is one criminal in a population of 5000, and a witness is 99% reliable, that still results in 50 false positives.
 * This problem is somewhat more recognized in medical practice. False positives from an apparently reliable test for a rare disease will swamp the actual afflicted ones, unless some kind of pre-screening removes most of the healthy. If the disease affects one in a thousand tested persons, and the test is 99% reliable in rejecting healthy persons, then the false positives will outnumber the diseased persons at least ten to one.
 * This happens every time you think you see an acquaintance who actually turns out to be a complete stranger. Can be embarrassing at parties.