Audio problem: When Bart's gang is singing their song and Nelson is playing the banjo, he sings "because she's dead" and plays different notes yet his hands don't move along the neck of the guitar.
Other mistake: When the Amish are building the tree house, the top of the tree is missing. It's just sky. In the next shot the top of the tree is back.
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