Treehouse of Horror XIV - S15-E1
Continuity mistake: The piles of sweets and candy that Bart and Lisa have at the start vanish in the wide shots when Homer arrives.
Treehouse of Horror XIV - S15-E1
Continuity mistake: When the younger Professor Frink is receiving and wearing his Nobel Prize, his father goes nuts and starts stealing people's brains. The younger Frink figures out that kicking the elder Frink in the crotch will stop him. When he does, and the elder Frink falls off the stage, he is wearing the Nobel Prize, not the younger. (00:15:00)
Treehouse of Horror XIV - S15-E1
Continuity mistake: After Homer tries to stab himself with the banana, he drops it. But when Nelson's clothes disappear, the banana has vanished.
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