Brother From Another Series - S8-E16
Sideshow Bob: You do know I had a...problem with trying to...kill people?
Cecil: [sarcastic] Goodness, I had no idea! For you see, I have been on Mars for the last decade, in a cave, with my eyes shut and my fingers in my ears.
Sideshow Bob: Touché, Cecil.
Brother From Another Series - S8-E16
Bart: Inside the body of every hardened criminal beats the heart of a ten-year-old boy.
Lisa: And vice versa.
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