Continuity mistake: The canister the hippies use to destroy the germs in Burns lab has the words "antibiotics" appear after the clock rings.
Continuity mistake: When Marge talks to cable guy, the trees and hammock disappear.
Continuity mistake: The moss on Walt Whitman's grave when Homer removes it changes.
Continuity mistake: The antenna from Homer's car disappears when he drops Mother Simpson at the gas station.
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