One Fish, Two Fish, Blowfish, Blue Fish - S2-E11
Continuity mistake: When Homer is given his plate of sushi, there are four pieces on his plate. Moments later, before he begins to eat, he has six pieces on his plate. (00:03:35)
One Fish, Two Fish, Blowfish, Blue Fish - S2-E11
Continuity mistake: Just after Homer says, "This fish is delish", one of the pieces of sushi keeps reappearing every time Homer eats it.
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