Continuity mistake: When Homer is talking to the makers of Police Cops, the person on the far left's hair changes from brown to grey instantly between shots.
Continuity mistake: The name card display stand on Homer's desk disappears after Mr. Burns interrupts him.
Revealing mistake: There are two Ruth Powers in the same shot, when the people are crowding on Homer at the mall.
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