Kill the Alligator and Run - S11-E19
Continuity mistake: When Homer beats the train they are on a road, but then another train comes by and hits them right where the road was.
Kill the Alligator and Run - S11-E19
Continuity mistake: When The Simpsons are being chased by the police, the cop is seen yelling at Homer. When the shot changes to Homer, the cop pulls out a gun from thin air and shoots.
Kill the Alligator and Run - S11-E19
Continuity mistake: The 2 guitarists in Kid Rock's band swap hair color when Kid Rock first arrives on stage.
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