Politically Inept, with Homer Simpson - S23-E10
Revealing mistake: Right before Homer is patted down at the airport, his facial hair briefly flickers yellow.
Politically Inept, with Homer Simpson - S23-E10
Revealing mistake: While Homer catches his underwear with his tongue after the Simpsons' clothes burst out of the suitcases, Homer's tongue briefly passes through his underwear.
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