Continuity mistake: When Homer is eating donuts while everyone is doing push-ups, his donut stays the same size every time he bites it.
Other mistake: When Moe is rolling his eyes after saying it was the first time he poured beer into the water supply, Herman's face is drawn incorrectly.
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