Bart Gets Hit By a Car - S2-E10
Continuity mistake: In Moe's Tavern, in the close-up of Marge as she says "Homer, look at me" the glass window behind her has a different pattern than in all the other shots (2 rows of red tiles instead of 3).
Bart Gets Hit By a Car - S2-E10
Continuity mistake: During Marge's testimony, Lionel Hutz has a neat stack of papers in front of him the whole time. But in the close-up when Homer and Hutz both cover their eyes in sorrow, Hutz has several pieces of crumbled paper and a broken pencil in front of him. The was not nearly enough time for him to make such a mess since the last shot of him.
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