Continuity mistake: As Homer begins his improv act, Lisa is seated elbow-to-elbow with Dr. Nick. In the long shot view from the rear, she is much further from Dr. Nick.
Continuity mistake: Lenny and Carl's seating in the auditorium changes from being down close to the stage to being higher up and further from the stage.
Continuity mistake: Before Homer begins his speech to the nuclear power plant employees, Lenny and Carl are wearing white/off white shirts with ties. After the speech, Lenny is wearing his usual green shirt with suspenders and Carl his usual gray jacket.
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