Urinetown ends with the heroes triumphantly toppling the evil toilet monopoly and launching a new era of free urination. Only, as it turns out, the monopoly was right about the water shortage, the newly unrestricted flushing makes the town run dry, and everyone dies of thirst.
Little Sally: What kind of a musical is this?! The good guys finally take over and then everything starts falling apart?! Lockstock: Like I said, Little Sally, this isn't a happy musical. Little Sally: But the music's so happy! Lockstock: Yes, Little Sally. Yes, it is.
The Wild Duck Nice job telling your friend his family's been lying to him all these years. And now his daughter's dead.
Wicked finds Elphaba listing her contributions to this trope in the song "No Good Deed".
Sweeney Todd: The Demon Barber of Fleet Street: If Anthony Hope didn't bust in Sweeney's barber shop in the midst of his shaving of Judge Turpin, his plans to rescue Johanna Barker would have gone according to plan, and Sweeney himself wouldn't have gone off the deep end due to being denied his first shot at revenge.
In the 2007 film adaption of Hairspray, when Tracy participates in the march and hits a cop on the head with her sign and runs away, it gives Velma a good opportunity to stop Tracy from participating in the pageant by having the same police guard the entrance, making Tracy and the others devise a plan to sneak into the pageant. If Tracy had been bailed out along with Maybelle, she wouldn't have had to hide from the cops and sneak into the pageant!
In Pokémon Live!, Pikachu's Thundershock and Thunder were the last two moves MechaMew2 needed to learn; with the list complete, nothing can stop Giovanni from conquering the world.