Continuity mistake: When Bart comes out of the water with the cooler, he doesn't have a shirt on. At no point does he put his shirt back on, it reappears when Lisa takes the food off everyone. (00:12:45)
Audio problem: When Bart takes Milhouse's inhaler to go after the cooler, Milhouse says "Hey, I need that to live". His mouth doesn't match with what he is saying. (00:12:15)
Continuity mistake: Just before Bart mentions the Swiss Family Robinson, Lewis is standing in front of him. When Bart mentions the Swiss Family Robinson, Lewis moves further back without enough time for him to move. (00:07:50)
Audio problem: When Bart and Nelson are having a fruit race on the bus, the kids mouths don't move when they are yelling. This occurs in the wide shot. (00:05:40)
Continuity mistake: In the shot before Nelson says "Hey, Simpson. Race you", Ralph is seen sitting on the row in front of Martin. But when Ralph says "Go, banana" he is suddenly sitting two rows in front of Martin. (00:05:25)
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