Continuity mistake: When Marge is putting Lisa and Bart's clothes on, she puts Lisa's dress on Bart and Bart's shorts and t-shirt on Lisa, but 2 seconds later, when they are entering the kitchen, they are wearing their normal clothes.
Continuity mistake: There are two problems with Lisa's Indian burns. When we first see them, she has 4 burns. Then, while she is showing them to the bullies, there are 3, then 4 again. Then, in the very next scene when she is walking with Nelson in the schoolyard, they are gone altogether.
Continuity mistake: When Lisa is watching tapes of Francine's bullying, there is a scene where they are at the pool. Francine is wearing nose clips in this scene. But when Lisa freeze-frames and leans in to closer inspect the shot, Francine's face is shown on the screen without nose clips. They appear again when Lisa exclaims "Of course. The nose clips."
Continuity mistake: When Marge is racing the school bus, as the bus jumps out of the ditch you can see that the bottom of the bus is painted to resemble a skateboard. When the bus lands from this jump, however, the bottom of the bus is an even grey colour.
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