Audio problem: At the theatre, when the cook talks to Mark Hamill off-stage he sounds exactly like Moe the bartender.
Audio problem: At the end, where Homer is carrying Mark Hamill out of the theater and Mark says, "Shoot, I forgot my Light saber", his mouth doesn't match what he says, and it keeps moving when he's quiet.
Audio problem: At the end where Homer is carrying Mark Hamill out of the theatre, Mark's lips don't match with what he's saying.
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