Marge vs. the Monorail - S4-E12
Other mistake: When Homer crashes his car, he just disappears when it hits the tree.
Marge vs. the Monorail - S4-E12
Other mistake: Lanley's desk in his trailer suddenly gets brighter even though none of the lights in the trailer were turned on.
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