Revealing mistake: After Apu and Manjula go by in horse and cart everyone returns into Moe's. When they walk in they just disappear into a brown wall which is where the inside of Moe's should be seen.
Revealing mistake: When Homer says "Oh, baby we got him now" whilst arriving in the airport, you can see half of Doctor Hibbert's face in the background, but you can see he doesn't have any eyebrows or eyes.
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