Continuity mistake: When Homer gets out of the electric chair, the sponge has disappeared from on top of his head.
Continuity mistake: At the end of the episode when Bart, Lisa and Maggie greet their parents Bart says that they were in the Green Room (a room where the cast of a show can relax when not performing) and comments that '[he] ate so much shrimp!' This conflicts with the episode 'Please Homer, Don't Hammer 'Em', where it is revealed that Bart is allergic to shrimp.
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