The Secret War of Lisa Simpson - S8-E25
Other mistake: When Lisa looks down from the platform before crossing the obstacle course, you can see her feet, but not the platform she stands on. (00:19:04)
The Secret War of Lisa Simpson - S8-E25
Continuity mistake: When Lisa and Bart are practising for the 'Eliminator', Lisa doesn't have a rope round her waist, but when she falls she does.
The Secret War of Lisa Simpson - S8-E25
Continuity mistake: When Lisa climbs to the top of the Eliminator, there is a platform for her to stand on. Strangely in the next shot Lisa is standing on nothing so she appears to be standing in mid-air.
The Secret War of Lisa Simpson - S8-E25
Continuity mistake: The vehicle they are driving changes form when they drop off Bart at military school to when they drive home. They leave with Homer's purple car (obvious from the crumpled headlight) and as they leave the military school it's Marge's orange car (obvious from the ski racks and lack of a trunk). It's clear from the entire scene that this is all one trip.
The Secret War of Lisa Simpson - S8-E25
Factual error: When Bart smashes all the glass in Springfield(as a result from the sound wave he made by speaking through the mega phones)the car windows get smashed as well. But when we see Chief Wiggum and Homer's car, the glass is still intact.
The Secret War of Lisa Simpson - S8-E25
Continuity mistake: When Lisa has just been saved from falling, from the Eliminator at night, three boys approach. The two boys on the right swap positions between shots.
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