Mistaken for Index

""I must be the only gullible husband who ever overheard snippets of surprise-party planning, and believed his wife was having an affair!""

- Homer Simpson, The Simpsons, "Eternal Moonshine of the Simpson Mind"

An episode based on a misheard conversation, often involving Innocent Innuendo.

A result of a principal character misinterpreting something. In comedy, this often leads to further and further misunderstandings, each more comical than the last, until things get straightened out at the end of the episode. In dramas, the principal character usually exerts much effort trying to prepare for a "showdown," only to discover at the last second it was "all a huge mistake."

This trope is one of the basic elements of Farce, but can lead to an Idiot Plot.

Sub Tropes

 * Abuse Mistake
 * Accidental Pervert
 * Actually Not a Vampire
 * And You Thought It Was a Game
 * Dude Looks Like a Lady
 * God Guise
 * I Am Not Weasel
 * Mistaken Age
 * Mistaken Confession
 * Mistaken for Aliens
 * Mistaken for Apocalypse
 * Mistaken for Badass
 * Mistaken for Cheating
 * Mistaken for Dying
 * Mistaken for Evidence
 * Mistaken for An Imposter
 * Mistaken for Junkie
 * Mistaken for Masturbating
 * Mistaken for Murderer
 * Guilt by Coincidence
 * Mistaken for Pedophile
 * Mistaken for Prank Call
 * Mistaken for Pregnant
 * Mistaken for Profound
 * Seemingly-Profound Fool
 * Mistaken for Racist
 * Mistaken for Servant
 * Mistaken for Special Guest
 * Mistaken for Spies
 * Mistaken for Subculture
 * Mistaken for Gay
 * Mistaken for Terrorist
 * Mistaken Identity
 * Mistaken Message
 * Mistaken Nationality
 * No Longer with Us
 * Not What It Looks Like
 * Poor Communication Kills
 * Real Joke Name
 * Suggestive Collision

Anime & Manga
"Evangeline: Hurry and whip it out, boy!
 * Mahou Sensei Negima! has a couple of these, most obviously when a bunch of the girls overhear Friendly Neighborhood Vampire Evangeline requesting that Negi pay for that day's training session, which goes something along these lines:

Negi: But Evangeline-san, we already did it, it's too much!

Evangeline: I told you, call me master."

"Chachamaru: I have been serving as Negi's partner every night and he seems to be happy about it."
 * By the way, the payment was sucking blood from his arm.
 * Later on, Chachamaru nearly causes Chisame's head to explode with this poorly worded statement about Negi's martial arts training.:

Comics

 * Used in an infamous storyline in the Popeye comic strip; A woman overhears Olive Oyl talking about getting rid of a baby robot a home shopping channel had mistakenly sent her and assumes she's talking about getting rid of her (unborn) baby and quickly assembles a crew of her cohorts to talk her out of it. Although there was little negative feedback from readers or newspapers, the artist behind this strip was soon fired (The official reason being that the artist had gone too far in trying to include modern elements into such a legacy strip. The "abortion" strip was merely the last straw).

Films -- Live Action

 * In the 1947 comedy Copacabana, Lionel and Carmen have made up a fake stage persona, Mlle. Fifi. When they decide to dump the persona, an old woman hears them joking about it and misinterprets them as saying that they murdered Fifi (who no-one else knows was just Carmen in a veil with a French accent). Hilarity Ensues.
 * A scene from Look Who's Talking! has James pulling out a splinter from Molly's finger. Her mother overhears and assume they're having sex. When James comes out, he zips his fly.

Literature

 * Literary example where a mis-heard conversation made a major difference in the story: in David Weber's short story "Nightfall" in Changer of Worlds, two characters are preparing evidence so that, if it becomes necessary to remove another character (Esther McQueen), they'll have backup. They spend some considerable time talking about the necessity of hiding this action, since they need McQueen and will for some time yet. The final comment of the conversation (approximately, "We'll need this when we pull the trigger on McQueen") is overheard and passed to McQueen -- where it triggers a full revolt. McQueen repeatedly complains that if she'd been given even six more weeks she would really have been ready. The revolt fails, McQueen dies, in the aftermath the government falls -- and the entire premise of the first 8-9 books in the series (good monarchy against evil socialist republic) is fundamentally altered. The series is up to 12 books now.
 * It should be noted the series was to this point Horatio Hornblower In Space with Esther McQueen being the expy of Napoleon. This is the story that goes off the plot rails.

Live Action TV
"Chandler: Oh, I think this is the episode of Three's Company where there's some kind of misunderstanding.
 * Just one of many from Threes Company (which made so much use of this that it was the Trope Namer at one point): Mr. Furley, the landlord, overhears Jack and Chrissy trying to hang a shower curtain in the bathroom, which begins with Chrissy saying something like "You'd better get into the tub with me so we can get it on" and "come on, I'll show you what to do". As Jack is permitted by the landlord to live with two women only because he is supposedly gay, Hilarity Ensues. Chrissy then falls, hits her head and is rushed to the hospital, and her roommates misunderstand the doctor and she is Mistaken for Dying.
 * This troper's absolute favorite example involved the episode when the main characters met the British Ventriloquist Leslie and his puppet Pamela. Because he kept Pamela a secret for certain reasons (she was kept in a large suitcase that he would not let anyone touch), several misunderstandings came out. First Pamela was mistaken by Jack as Leslie's crime partner after reading a news article on the "Duke and Duchess" (Leslie was mistaken for being the "Duke"), then she's mistaken for being Leslie's girlfriend when Jack lets Janet and Terri hear Pamela's (actually Leslie's) voice through his bedroom wall. The Crowning Moment of Funny of the episode was a case of Mistaken for Murderer when the trio break into Leslie's apartment, and Jack opens the suitcase thinking that's where Leslie kept the stolen money in, only to pull out Pamela's hand. Cue the trio screaming in terror and fleeing the scene in the most hilarious fashion.
 * This was lampshaded in an early episode of Friends (who would later still use this trope plenty anyway):

(studio audience laughs)

Pheobe: Well then I've already seen this one. (turns off TV)"

"Jonathan: How many men left?
 * There is a persistent rumour of an unaired Frasier episode that's not based at least partially on this plot (or the "A Simple Plan" plot).
 * After some research, it has finally been found. It was "War of the Words" from the show's 9th season.
 * Hilariously enough, "War of the Words" is the lowest-ranked Frasier episode of all time on IMDb. Another sign that Tropes Are Not Bad and that shows should stick to what they're good at.
 * In the "Death of a Bailiff" episode of Night Court, Bull gives away all his possessions after a near-death experience in which he thinks God said to him "Give to the poor, and thou shalt have riches in heaven." It was actually a person Bull was feeding cable to, who said over a walkie-talkie "Give me some more; I'll shout when it reaches eleven."
 * Coupling does this a bit, too: Sally approaches Patrick's bisexual girlfriend, trying to get a confession that she "fooled around" with Jeff (when no such thing had occurred), and the girlfriend thinks Sally is hitting on her; Jeff talks about how Jane's clinginess to Steve means "he's got an unflushable", and Susan, who just met Steve in the bathroom, thinks it means something else entirely.
 * In another episode, Steve and Susan are watching a TV show which mentions the number of men who continue to masturbate when in a committed relationship. In the awkward silence that follows, Steve starts whistling, in an attempt to seem relaxed. He justifies it by saying that he felt like some music but wasn't in the mood for a whole CD. "Sometimes you want a full orchestra, and sometimes you just want a... quick whistle?" Susan tells him that she doesn't mind his whistling, as long as he doesn't get "whistled out." Later in the episode, Susan's parents come round for a visit, and Susan tells them about how Steve has started whistling to himself. Her father brings it up with Steve, referring to it as "going solo," and Steve interprets it how you'd imagine. This leads to the classic line, "If music be the food of love, then masturbation is just a quick snack between meals." who suddenly start talking about whistling. Steve eventually throws them out after her dad said that he shouldn't whistle to much or else he'll be too tired to pucker. Which he misheard for... well, you know.
 * Justified in an Angel episode. Cordelia is magically shown several conversations her teammates have about her by the demon Skip, all of them seemingly very insulting towards her. However, Skip is actually deliberately showing her very specific parts of the conversations taken out of context for his own agenda.
 * Averted in Little Mosque on the Prairie: Fatima overhears a conversation between Rayyan and JJ -- "why not do it now, we're gonna do it after the wedding anyway..." "after the wedding, I want to do it right in front of my parents!" -- and correctly guesses that they're talking about when they should open their wedding presents.
 * Every episode of the German series Hausmeister Krause. Yes, all of them.
 * The out-of-theater plot to the Mystery Science Theater 3000 episode Mitchell revolves around this. The Mads have hired Mike Nelson to help with an inventory of the Deep-13 lab beneath the Gizmonic Institute, but they find him insufferable and decide to kill him. Gypsy overhears them plotting and comes to the conclusion that they're plotting the death of Joel and spends the rest of the episode plotting to help Joel escape the Satellite of Love. Thus did Joel leave the series and was replaced by Mike
 * Happens during Earl's coma fantasy in My Name Is Earl. Earl is the star of a 1950's style Dom Com in his head, and he is married to a friend's ex-girlfriend that Earl was attracted to. She is pregnant, and conversing with Joy (their next-door neighbor) about a really awesome guy. Earl thinks she's talking about a gigolo...it turns out she's referring to a doctor.
 * In an episode of Jonathan Creek Maddy does this deliberately to Jonathan, interrupting their phone conversation with lines like "Oh, go on! Take me! I'm powerless to resist!" He isn't fooled:

Maddy: Four pawns and a bishop.

Jonathan: Resign."

Web Comics

 * PvP, which calls back to seventies shows often, does this quite a bit. Subverted in that Cole hears Brent and Jade having sex in their office, realizes he's probably making assumptions like he has lots of time before, and comes to the conclusion that they're just moving furniture.
 * One of the page quotes alludes to the brief, one-sided relationship that tried to take root while Elan was separated from Haley in Order of the Stick. Although he's aware of the trope (Vaarsuvius notes early on that Elan's training as a bard makes him very Genre Savvy), he's generally compelled to let tropes rule in the name of narrative even if acting on his insight would save a lot of headaches later (he once delayed Roy during an escape from a self-destructing dungeon because they escaped with several seconds to spare, and had to wait for the dramatic fireball to catch up).

Western Animation

 * Kim Possible: Ron Stoppable breaks into his girlfriend's house, then her closet, steals her super battle suit, joins the football team as star quarterback, gets caught with the suit and controlled by a villain, ends up in a physical and emotional fight with Kim, then ends up on the team anyway (and is still a star player just in a different position), all because he thought Kim was going to take Bonnie's advice about "trading up" to a socially acceptable jock boyfriend. Ron then overheard Kim talking with Monique about trading up and agreeing with Bonnie. Turns out they were talking about a new mobile phone.
 * The Thomas the Tank Engine episode Percy's Big Mistake had Percy overhearing The Fat Controller say something about scrap and thought it meant he was to be scrapped (he's a steam engine)..