Revealing mistake: When Moe grabs Ned Flanders' shirt at the bar, Ned's torso is incorrectly drawn over his arms as well as the mug he is holding. (00:10:50)
Revealing mistake: The people waiting in line for the game are extremely crude drawings. Some of them aren't even completely drawn, e.g, some are missing arms, legs and other body parts.
Revealing mistake: In one angle when the Flanders are having dinner, Ned's eyeglasses have no temples.
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