Last Exit to Springfield - S4-E17
Mr. Burns: Who's that man?
Smithers: Homer Simpson, sir.
Mr. Burns: Simpson, eh? New man?
Smithers: Actually sir, he thwarted your campaign for governor, you ran over his son, he saved the plant from meltdown, his wife painted you in the nude...
Mr. Burns: Ehh...doesn't ring a bell.
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