The Old Man and the Lisa - S8-E21
[Mr. Burns is grocery shopping for the first time and sees Krusty getting a box of Krusty O's]
Mr. Burns: Could you tell me where I might find the Burns O's?
Krusty: Sorry Pops, they don't put nobodies on cereal boxes.
Mr. Burns: [Looking at a box of Count Chocula] Well, I suppose this one looks a bit like me.
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