Tree House of Horror V - S6-E6
Audio problem: In 'The Shinning' when the Simpsons finally arrive at the house, they climb out the car and start yelling "Hi." and "We're here." Listen to Lisa's voice, and watch her mouth. The audio is totally out of sync with her mouth movements. (00:02:30)
Tree House of Horror V - S6-E6
Audio problem: In 'Time and Punishment,' Lisa yells "Dad your hand is jammed in the toaster!" and we then hear her screaming in the following shot, however her mouth is closed. (00:08:55)
Homer Badman - S6-E9
Audio problem: When Bart and Lisa hug the T.V and Homer asks them if they are, they respond with "No". However, Lisa's mouth moves but you hear no sound.
Audio problem: When McBain is explaining about his movie on TV, we see a clip from his movie, in which he uses a machine gun. He throws it on the floor but we don't hear a sound of it crashing on the floor.
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