Treehouse of Horror XXI - S22-E4
Continuity mistake: In the second segment, after Roger has told them about what happened to him, a radio appears out of thin air.
Treehouse of Horror XXI - S22-E4
Continuity mistake: In the second segment, as Roger is telling them about what happened to him, as he finishes the story of Marge and Homer saving him, in his recount Marge is wearing a shirt and bikini bottoms (the shirt is covering the bikini bra). But when they saved her, she was just wearing a bikini.
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