Marge vs. the Monorail - S4-E12
Plot hole: When Homer is trying to stop the Monorail, he grabs the big M from the side of the train and uses it as an anchor. However, when the M cuts through 'Springfield's Oldest Tree', there is no visible monorail track or supporting pole in the background.
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