Sweet Seymour Skinner's Baadasssss Song - S5-E19
Principal Skinner: I finally have time to do what I've always wanted: write the great American novel. Mine is about a futuristic amusement park where dinosaurs are brought to life through advanced cloning techniques. I call it "Billy and the Cloneasaurus."
Apu: Oh, you have got to be kidding, sir. First you think of an idea that has already been done, and then you give it a title that nobody could possibly like. Didn't you think this through? [The scene jumps forward in time.] It was on the best-seller list for 18 months. Every magazine cover had it... [The scene jumps forward in time again.]...one of the most popular movies of all time, sir. What were you thinking? I mean thank you, come again.
Answer: I'm not sure if this is the one you're thinking of, but an episode of "Ray Bradbury Theatre," called "A Sound of Thunder," dealt with a similar matter: a group of hunters travel back in time to hunt dinosaurs, only to find things have changed when they get back because someone stepped on a butterfly.
Xofer