Audio problem: When Marge says "Now that it's empty, who's going to buy their house? What if someone moves in with two Barts? Or four teenage Barts?" her mouth movements don't match most of her dialogue.
Plot hole: Warren's (as Bob) face falls off and tries to hold it with his hand. When he appears in the next scene, his face is totally sealed with nothing to seal it with. (00:16:25 - 00:17:55)
Continuity mistake: Bob duct tapes Bart's mouth before telling him his story, but the duct tape disappears when the story finishes. (00:12:30 - 00:14:10)
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