Continuity mistake: When the family are talking and Marge is sitting on the sofa, Bart runs and shoves Lisa (who is in front of Marge) out of the way, but in the next shot, she is still in front of Marge, despite the fact Bart pushed her out of his way.
Continuity mistake: When the Simpsons are driving to Cypress Creek the arrangement of luggage on the roof of the car changes depending on their stage in the journey.
Continuity mistake: When Homer is walking and talking with Hank Scorpio when he first arrives at work there is reversed "J" on Homer's tie that comes and goes between shots.
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