Continuity mistake: In the cafeteria at the power plant, in the wide shot the clock on the wall is on 9.00, but closer up it's showing 10.35.
Continuity mistake: The hole in the wall made by the acid changes shape and size in several scenes.
Continuity mistake: Lenny says that Homer got his job by showing up when the nuclear plant was opened. In the episode "I Married Marge" (Season 4), the plant is already open when Homer interviews for a position, doesn't get it, then works at Gulp 'N' Blow.
Continuity mistake: After Milhouse explains to Bart how the warehouse collapsed, he appears to be bald for some reason.
Revealing mistake: When Lenny is talking about Homer not needing a degree to work at the power plant, Lenny's five o'clock shadow briefly detaches from his face.
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