Continuity mistake: Near the start when Marge is putting jam on bread, she spreads it on the last piece but as she puts it on the plate it changes to a blob in the middle.
Audio problem: When Homer's vigilante group are putting out a leaf fire, Homer says "No burning leaves without a permit" his mouth is not moving.
Revealing mistake: When everyone is cheering for Ned at the neighborhood watch meeting, Apu's hand is yellow.
Revealing mistake: When Homer is on the news talk-show talking to the cat burglar, he puts his hand on his chest and the opening of his shirt appears over his hand.
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