Premature Aggravation

"[T]he Imaginative Man who works himself into a Passion because he thinks Some one may insult him."

- John T. McCutcheon, writing the Laconic Wiki entry a full century before it exists

Alice and Bob are going to meet each other at some point, to negotiate something or just make amends with each other. While Alice's waiting for Bob to show up, various horrible cases of the What-Ifs start creeping into her mind, making her more and more angry.

By the time Bob shows up, Alice's gotten herself so worked up that she blows up at the fictional events that have occurred in her mind. Usually before Bob even has a chance to open his mouth to say anything beyond "Hello."

Film
"President Rufus T. Firefly (Groucho Marx): I'd be unworthy of the high trust that's been placed in me if I didn't do everything in my power to keep our beloved Fredonia in peace with the world. I'd be only too happy to meet with Ambassador Trentino, and offer him on behalf of my country the right hand of good fellowship. And I feel sure he will accept this gesture in the spirit of which it is offered. But suppose he doesn't. A fine thing that'll be. I hold out my hand and he refuses to accept. That'll add a lot to my prestige, won't it? Me, the head of a country, snubbed by a foreign ambassador. Who does he think he is, that he can come here, and make a sap of me in front of all my people? Think of it-- I hold out my hand and that hyena refuses to accept. Why, the cheap four-flushing swine, he'll never get away with it I tell you, he'll never get away with it! (Trentino (Louis Calhern) enters) So, you refuse to shake hands with me, eh? (Firefly slaps Trentino with his glove for the second time in two days and the war begins)"
 * From the classic Marx Brothers comedy Duck Soup:

Live Action Television
"Herbert: "Oh, hello Ernie!" Ernie: "Oh yeah?! Well if that's how you feel you can just keep your stupid old vacuum cleaner, so there!""
 * In one Sesame Street sketch, Ernie wants to borrow a vacuum cleaner from Herbert Birdsfoot, but goes through this while imagining Herbert's possible reaction to the request. When they finally meet:


 * There was a similar story in a picture book called Some Friend!
 * On Scrubs, J.D.'s inner monologues sometimes lead to this.
 * Two memorable ones: First, there was the time he went into an Imagine Spot to hypothetically talk to people who died because of him, so he could ask them whether they thought he killed them. This imagine spot takes place in a diner, and when he orders flapjacks discovers heaven doesn't serve them. He spends the rest of the episode calling flapjacks 'devil-cakes'.
 * Secondly, he once imagined Turk being a father, and accidentally taking a pumpkin home instead of the kid. Eventually, the pumpkin graduates school, but falls and splatters open, then Turk's biological son gets hit by a car. When J.D. comes out of it, he calls Turk a terrible father.
 * On Dead Like Me, George gets a dog (J.D.) and is kicked out of her apartment because Daisy is allergic to dogs. George moves in with Mason, but while at work is overcome by an irrational fear that Mason would bring J.D. into a hot tub and use him to pick up chicks and promptly moves out. George later chastises Mason for this imagined slight, leaving Mason confused.

Newspaper Comics

 * The comic strip linked to in the page quote is from 1905.

Web Original
"Batman: ... and I'm Batman. Iron Man: [Snapping] Oh, fine, yes! Okay? I know! Alright?! You're Batman! You're awesome! Fine! I get it! Alright?! Huh? Huh?! Huh?! [Storming off] GOD!!!! Batman: [Bemused] ... Fine, I'm a DC."
 * In the seconds before doing an I'm a Marvel And I'm a DC skit with Batman shortly after the release of The Dark Knight, Iron Man finds himself sympathizing with Batman about how the movie will never live up to the hype that's built up around it... only, after saying his familiar "Hi, I'm a Marvel," line, suffering through a dream sequence in which it does, and more besides. As a result, after Iron Man faces an ordeal with hyperactive fans, time travellers from a future where the success of The Dark Knight has resulted in a utopia, an alien species opening trade relations based on the movie's wonderfulness, Santa Claus skipping Christmas to catch it and Jesus putting the Rapture on hold just so that he can catch the twelve o'clock screening, he comes back to reality:


 * By next episode, it turns out he's not entirely wrong.
 * Almost a Funny Aneurysm Moment after the success of Iron Man, eh?

Web Comics

 * This Xkcd strip combines this trope with Super Window Jump.

Western Animation

 * Homer Simpson does this with some frequency. Oddly enough, they usually spur him to do the right thing, although it is almost always for the wrong reasons. One memorable example has Homer initially unconcerned about Bart's escaping from juvenile hall- his emotion is ratcheted up to alarm once his mental hypothesis concludes with Bart being forced into a shotgun wedding to an alien.
 * An episode of The Brak Show has Brak's father declaring a hypothetical brother dead to him after imagining what he would be like.