Continuity mistake: When Lisa meets the other kids for the first time under the boardwalk, her tinted shades disappear in the very last shot.
Continuity mistake: When Marge is cleaning the dishes on the morning after Bart reveals Lisa as a nerd, Marge is seen in close-up with a white necklace. In next shot of Bart and Lisa, as well as the rest of the scene, Marge has a red necklace.
Factual error: Bart uses Milhouse's glasses as binoculars, but as Milhouse is near-sighted this wouldn't work.
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.