Continuity mistake: The lock only appears on the door when Marge uses it. (00:14:45)
Continuity mistake: When the two chinese guards are dragging Budda Homer along, in an overhead shot we see a desk, with one filing cabinet behind it. In the next shot, the guards stop to rest, and now there are two. (00:17:25)
Continuity mistake: After Selma gives Madam Wu the sack of ten thousand "happinesses" the sack instantly vanishes from off the table. (00:08:40)
Continuity mistake: When Cletus arrives, to get his child back from Selma, we see Lisa wearing purple gloves. In the following shots, as she speaks to Selma, her hands are bare. (00:04:40)
Continuity mistake: The hospital band on Selma's right wrist keeps appearing and disappearing after they watch the menstration video. (00:03:00)
Continuity mistake: In the first shot of Selma on the hospital bed, there's nothing behind Patty. However, when Marge starts to speak, we suddenly see two bags of drip mounted on a dripstand behind her. (00:01:55)
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