Other mistake: When Bart, Lisa and Milhouse run away from the others, Lisa steps right on a puddle but no splash or ripple is made.
Other mistake: When everyone is eating the boar at the end, the boar is completely intact, yet everyone except for Lisa is seen with a piece of it. (00:20:30)
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