Gal of Constant Sorrow - S27-E14
Continuity mistake: When Homer returns from work to get the cat out of the plug socket, he removes the plug socket cover without removing any of the plugs, but when Homer realises that the cat is in the ceiling, both the plug socket cover and the plugs have now been removed from the wall.
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