Pygmoelian - S11-E16
Continuity mistake: When Moe is reading the script there are no pink pages in the book, but when the director says "pink pages are dreams" they show pink pages that weren't there before.
Pygmoelian - S11-E16
Continuity mistake: When Moe is going to get a plastic surgery, the surgeon draws some marks on his face. You see Moe from a side view, and the marks on his face aren't visible from that angle. However, the marks on his face suddenly appear and you can now see them from that angle.
Pygmoelian - S11-E16
Continuity mistake: At the start of the episode, Homer wakes the family up and they get in the car in their night clothes. When they arrive at the destination, they are dressed.
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