Lisa's Pony - S3-E8
Audio problem: When Homer is crying after the tape is finished, what he says doesn't match his mouth. (00:07:25)
Lisa's Pony - S3-E8
Audio problem: When Lisa rides past on her pony, Ralph is sitting on the fence with another boy who says, 'She really tamed that horse', Ralph says, 'Yes but who can tame her?' However he speaks with a deeper voice as opposed to his higher squeaky voice, and sounds exactly like Nelson Muntz. (00:15:55)
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