Other mistake: When Ned and Homer's raft is going fast, Ned decides to go on the left side of the gorge, so he paddles on the left side, which causes a raft to turn to the right, which 1, means the raft moved to the left, which is impossible, and 2, he wanted to go to the left, but paddled like he wanted the raft to turn right.
Continuity mistake: After Bart and Milhouse drink the all-syrup squishee, the syrup disappears from Bart's face between shots.
Factual error: As Bart, Homer, Ned and Todd drift away from land in their boat, we see the clouds moving away. The clouds would not move that fast at the speed that the boat was going.
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