Continuity mistake: At the end of the episode in church when the group of parishioners moves forward to forgive Ned there is a woman amongst the group who has light green hair. However, when there is a wide angle shot of the group again she is nowhere to be seen.
Continuity mistake: When Homer drives his car to work it is lime green instead of the usual purplish color.
Continuity mistake: When Ned is saying grace, the plates on the table contain no food, but when Homer comes through the window there's food on the plates.
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