You Know What You Did

A common soap opera scenario: young couple Alice and Bob are very happy. But Vamp Dorothy, who wants Bob herself, "casually" mentions to Alice that Bob spent the night with her (when in fact they were simply both stranded in a blizzard with thirty other people).

Given a choice between believing her beloved, who has never lied but always been faithful and true, or believing the scheming bitch with a history of deceit and an obvious agenda, naturally our dumb heroine immediately accepts the worst of the story, and totally rejects her true love without even asking his side of it. When he pathetically asks why she's upset, she only says, "You know what you did!" with a side of Death Glare, thus cementing what should be a Discredited Trope, but, sadly, is not.

At times the woman will ask the man some transparent question, such as, "Did you spend the night with Dorothy?" Unfortunately, the man doesn't actually get why she's asking, and in responding with detail-free answers - or answers with the wrong details ("Why, yes - we got a lot of exercise that night. Good thing, 'cuz I was getting pretty cold.") - merely confirms the woman's fears. The woman, of course, never asks clarifying questions that would resolve the matter, and the man never picks up on the fact that he needs to clarify because he does not speak nonverbal - in fact, sometimes he'll believe that the woman knows exactly what's going on to begin with, hence the lack of necessary detail.

And of course, the woman never wonders why, if he was cheating on her, he'd just casually admit to it like that, being that he hasn't displayed Jerkass traits before.

A form of Genre Blindness, with a giant, gooey Idiot Ball.

Anime/Manga

 * This is one of the problems in Akane Tendo's relationship with Ranma Saotome, of Ranma ½. She is willing to see the worst of anything slightly suspicious that Ranma does, something Ranma has actually lampshaded in the anime, jump to perverse conclusions about Ranma's goals/intentions/motivations, has been shown to consciously ignore elements of Ranma's own attempts at self-defense to prove her point and selectively remember events to further back up her opinions (manga version of Hinako's introduction), and believe the lies of people like Shampoo and Ukyo... who, by the way, are her devout rivals for Ranma's hand.
 * She isn't the only person in Ranma ½ who does this, though... unusually, however, the other people who do so tend to be Ranma's male rivals, using this trope in relation to their particular love interest being upset and them blaming that fact on Ranma (and using it as an excuse to try and beat the hell out of him). A good example would be Ryoga and Kuno attacking Ranma after he "kissed" Akane during the first Nekoken story, an event that Ranma doesn't remember due to "Kitty Ranma" being a kind of split personality.
 * Never mind that fact that it was Kuno who awakened "Kitty Ranma" in by exposing him to his fear of cats, but is too proud and stupid to take responsibility for the incident.
 * Fushigiboshi no Futagohime, ep 24 has bird people being teed off at the Windmill Kingdom and setting up barricades, and all one particular messenger will say is: "Why did we take such actions? King Randa (of the Windmill Kingdom) should know the reason." Naturally, King Randa is quite clueless. Al1701 says in his review: "What adult would accuse another adult of doing something, then refuse to say what they did?"
 * Tokyo Tribe 2 has this with the reason why Mera wants to kill Kai. For the first several episodes, we don't know what it is, with Mera and Skunk hitting Kai with these lines early on, in flashback and in present time. As it turns out in episode 12, what Kai did was actually

Comic Books
"Piggy: If you don't know why I'm mad, there's no point in me telling you, is there? Kermit: That doesn't make any kind of sense!"
 * In The Muppet Show Comic Book, Miss Piggy believes Kermit is seeing someone else, based on a cryptic and entirely fraudulent prophecy.

"Catwoman: How did you know?! Trickster: Er... know what? Catwoman: Don't play games with me! Trickster: Please, God, oh please, help me figure out what she's talking about... Catwoman: You... you just... made it all up...? Trickster: Can I get a category here? It would really help me defend myself if I knew what it was that you were going nuclear over..."
 * A nonromantic variant gets played with in one issue of Catwoman. The Trickster phones in a tip to the media that gives away her identity as mayoral candidate Selina Kyle, and she flies into a rage at him for outing her--and then it turns out that it was meant as a generic political smear campaign and the identification was a coincidence.

Film -- Live Action

 * Present in the The Lord of the Rings films (but not the books). It drives a wedge between Frodo and Sam during their journey through Mordor.
 * Subverted in the movie Thirteen Going On Thirty. The Alpha Bitch character tells Jennifer Garner's love interest played by Mark Ruffalo a nasty lie about her to get rid of him. Later we find out that while he has indeed been avoiding her, it's been for other reasons. ("It doesn't matter what Lucy said. I stopped trusting her after she stole my poprocks in the third grade.").
 * Subverted nicely in the Will Smith film Hitch, where the object of his affections took the word of a known Smug Snake, with every reason to lie, and proceeded to break up with Hitch without explanation, then trash him in her gossip column, before confronting him at a restaurant one night. Hitch, after pausing for shock, calls her out on this and corrects her loudly in front of everyone, leaving her looking quite the fool.
 * Then the film follows up with having her apologize and him not accept it, thus making the true climax him publicly apologizing for not accepting her apology, and begging her to come back to him.
 * White Christmas. A busybody hears part of a story that makes her think the male leads are planning to exploit their old friend. She tells everyone, including the female romantic interests, who almost leave the men over it.

Literature

 * This goes as far back as William Shakespeare. In Othello, Iago's plan only succeeds because Othello decides to believe the jealous subordinate over his beloved Desdemona, and he never tells anyone what he's angry about, only that he's angry. The rest of the cast never even thinks of asking someone other than Iago about what's going on. In King Lear, Gloucester decides to believe his embittered bastard son's claim that his legitimate son is a traitor, without thinking about motives or checking with anybody else.
 * In Othello, it's not that simple. Iago is known by all the characters as "honest Iago" because he has a reputation for always telling the truth, being seen as incapable of doing anything other than tell it like it is. Add in that Iago and Othello have fought in battles many times before to the extent that Othello has built up an absolute trust in Iago, who was always with him in the heat of battle. Though Othello does not ask Desdemona about this, Iago has a hand in this, saying first that Desdemona would just deny it and then that Desdemona deceived her father to marry Othello, when Othello questions why Desdemona would betray him when she has such a loyal character.
 * Also done in the play Much Ado About Nothing, but with the genders switched round. Hero and Claudio are to be married the next morning- but the evil Don John convinces the court that Hero has been sleeping around. The entire court (except for the heroes) immediately believe the deceitful Prince that previously tried to overthrow the court. Of course, in fairness, they thought they'd actually seen Hero in the act...

Live Action TV
"Marsha: If you don't tell her, then I will. Tim: But you'll spoil the surprise! Marsha: You bastard."
 * In the Monk episode "Mr. Monk and the Captain's Marriage", a police sergeant tells Captain Stottlemeyer that he's having an affair with Stottlemeyer's wife, and the captain is suspicious enough to have his wife followed, despite her protestations of innocence. And then he's surprised when she asks for a divorce.
 * Though it also works the other way: Stottlemeyer's wife was planning to divorce him before any of that even happened, and flat-out refused to explain why when he asked. Given that the audience saw virtually nothing over the series to back her up, it made her look like quite a bitch.
 * Stottlemeyer explains it himself to Monk in a later episode, the one where Monk gets shot. He's acting like a tremendous Jerkass to Natalie because he blames her for his injury, and she accepts it because she blames herself, and so due to essentially working round the clock taking care of Monk, wheeling him around in a wheelchair, etc., she is very nearly broken down. Stottlemeyer takes Monk aside, warns him that if he keeps acting like that Natalie will leave, then proceeds to state that he knows this because Monk is acting like Stottlemeyer himself and that that was why his wife left him.
 * This happens to Dexter during his second season. His girlfriend overhears Dexter's Narcotics Anonymous sponsor, who she's already suspicious of, mention on his answering phone that they spent the night together, and walks out on him. It's true...except it just means he fell asleep with his head in her lap after talking about his terrible childhood. She is his counselor, of sorts, after all. It's not actually shown if it was on purpose but considering how Lila turns out it would make complete sense that she did it on purpose knowing Rita would be there.
 * However, Dexter has just recently revealed that he is a drug addict and he has been lying to her about things to hide his addiction. So it is not much of a stretch for her to believe that he would cheat on her as well. At least she does not find out that he is lying about being a drug addict and is actually a serial killer.
 * When Rita finally confronts Dexter about it, he answers truthfully: he didn't have sex with Lila that night.
 * Thomas Riker uses this to his advantage in the Star Trek Deep Space 9 episode "Defiant". Realising that Chief O'Brien would be likely to spot that he wasn't his identical twin Will Riker, he acts snooty around him saying "You know why!" O'Brien defers to Riker's superior rank and leaves him alone - which is exactly what Tom wanted.
 * Subverted in Veronica Mars, when the Alpha Bitch claims that she and Logan had had sex. True to trope, Veronica believed this unquestioningly, and confronted Logan about it. They had.
 * Tim and Marsha get into one of these in the penultimate episode of Spaced. Marsha, who believes Tim and Daisy to be a couple, has seen Tim kissing his new girlfriend and confronts him about cheating on Daisy, threatening to inform her. Tim, however, does not know this, and because Marsha does not actually explicitly say anything about what she's seen, believes that Marsha is in fact referring to a birthday cake Tim has arranged for Daisy as a nice birthday surprise. Complications, naturally, ensue.


 * Averted in Chinese Paladin 3. When the heroine, who usually has some justification for her suspicions, angrily confronts the hero with this trope, he very sensibly replies "No, what did I do?" and explains what was going on. It's almost beautiful.
 * Beckett confronts Castle the morning after he broke the rules and saw a murder suspect that he had previously dated. She's talking about him messing with her chair. He's talking about kissing the suspect.
 * Sketch comedy show Almost Live addresses this in a sketch about a game show called "What The Hell Have I Done Now?" wherein a confused husband tries to figure out why his wife is mad at him.

Newspaper Comics

 * Parodied in This Dilbert strip.

Stand-Up

 * Used in a stand-up comedy bit by Adam Ferrara. "You know what you did," his girlfriend says. "No, seriously, what's-" "Well, if you don't know, know, than I'm not going to tell you!" The retort? "FINE! Then don't be surprised when this shit happens again!"
 * Drew Carey cites this as the reason a world ruled by women would not be peaceful at all, and talks about a situation in which one country suddenly invades another in such a world. The leader of the invaded country calls the leader of the invading one, and asks why they're attacking. The answer? "Oh, I think you know why!"

Webcomics

 * Subverted in a subplot of the webcomic Dominic Deegan. Manipulative Bastard Neilen Everstar tries to convince the protagonist and his girlfriend, respectively, that the other is discontent with the state of their otherwise happy relationship. Dominic being a seer invalidates the first half, but given Luna's propensity toward self-loathing, one would almost expect his ploy with her to be preordained to succeed. However, this isn't the case.
 * A purely accidental version occurs in Sequential Art, where Kat slumps face-first into Art's crotch while the two are sleeping on the couch, and accuses Art of being a pervert when she wakes up to find herself in said compromising position, leaving Art confused by her outburst since he was asleep when it happened. However, she learns this little detail from Pip soon after and apologizes to a still-confused Art.

Western Animation

 * Codename: Kids Next Door "Operation Dogfight" doesn't actually use the phrase, but it's a YKWYD plot anyway, and one that works to boot: some kid who has shot down Numbuh 2 several times walks in with a chili dog into a hobby shop where 2 is shopping and gives 2 a Death Glare. At first go, one is led to conclude that he's being all "Hey, I'M the new king of the sky now, so stay out!"
 * Said in one episode of Foster's Home for Imaginary Friends but quickly subverted as the character who said it was joking around.
 * An episode of Jimmy Two-Shoes had Beezy being haunted by a ghost. When he tries to figure out why, the ghost simply replies "You know what you did."
 * In an episode of Doug, the title character manages to destroy a condemned house with a single rock to impress Patti. However, this causes her to hate him for the duration of the episode because  The episode is exacerbated because Patti (and later Bebe, who asks her about it on Doug's request) refuse to pass that information onto Doug.
 * Parodied in Drawn Together. A housemate will make a disparaging remark about Tori Spelling, to which another housemate will ask "why you dissing on Tori?" The response is "she knows what she did" followed by dramatic background music.
 * In that cartoon where an attempt to turn The Thing of the Fantastic Four back into human turned him into a teenage boy, he attended school and there was one episode where the bullies framed him and, when he asked the teacher why she'd want to talk to him, she said something among the lines of "As if you don't know."

Real Life

 * When Activision fired two Infinity Ward employees for insubordination, they said something to this effect.
 * Pretty much everyone alive has a story of being confronted by a friend or loved one only to be given this stock line when asking what they did wrong.
 * Similarly, children are also subjected to this trope by their parents or teachers when they do something wrong, even if they honestly don't know what they had done wrong.