Bart's Friend Falls in Love - S3-E23
Milhouse: Bart, I don't want you to see me cry.
Bart: Oh, come on, I've seen you cry a million times. You cry when you scrape your knee. You cry when they're out of chocolate milk. You cry when you're doing long division and you have a remainder left over.
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