Large Marge - S14-E4
Factual error: In Luigi's, the lobsters in the tank are bright red. They are actually quite dark brown until they are boiled when they become red/pink.
Large Marge - S14-E4
Continuity mistake: After Homer paints "El Homo" on the wall, he paints over it so that none of the letters are visible. In the next shot a couple of the letters are visible, and are only struck out with one line.
Large Marge - S14-E4
Continuity mistake: In some shots, the incorrectly-colored lobsters have their claws taped shut, and in others they don't. It's completely random.
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