Tempting Fate/Theatre

In Little Shop of Horrors, much ado is made about the fact that Audrey and Seymour are getting married tomorrow. There's even a Cut Song called "We'll have tomorrow". They're both dead by the end of the day.
The Stage to Screen Adaptation of The Producers has a song about it being bad luck to say "good luck" on opening night. Therefore, everyone wishes Franz to "break a leg" as he dashes off to the show. It's just his luck that a moment later he literally breaks his leg.
- Also invoked by Max, who wants the show to fail and so wishes "good luck" to everyone he can. He even breaks a mirror and throws a black cat across the stage door path for good measure.
In Avenue Q, the number "If You Were Gay" begins with Rod reading alone in his apartment, saying, "No roommate to bother me, how can it get any better than this?" Cue "Oh, hi Rod!" as the door slams.
In Chicago: "And I said, 'You pop that gum one more time'... sigh... and he did. So I took the shotgun off the wall, and fired three warning shots. Into his head."
The second act of The Phantom of the Opera opens with a Masquerade Ball where everyone is having a good time and several principal characters express relief that the Phantom has been unheard from for several months. Yeah, guess who crashes the party and immediately mocks them for thinking they were going to be rid of him that easily?
"The Ides of March is come."
- "Ay, Caesar, but not gone."