Sex, Pies and Idiot Scrapes - S20-E1
Continuity mistake: When Homer has Snake cornered, the second rung on the ladder behind them is broken. In the next shot it's not broken, then it is again.
Sex, Pies and Idiot Scrapes - S20-E1
Continuity mistake: During the St. Patrick's Day parade, Bart is wearing a green shirt throughout the whole scene, but he's wearing his usual orange shirt when he says "Where's the IRA when you need them?"
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