Continuity mistake: When playing Monopoly the fold in the board is level with the P. Camera changes now the fold is before the O. (00:16:10)
Continuity mistake: Marge tells Bart not feed Maggie the Monopoly houses. Bart shows the box and says that there's enough of them. The shot later, the box has disappeared. (00:16:15)
Continuity mistake: Maggie spits a hotel out on to the board which disappears a few seconds later. (00:16:15)
Audio problem: After Barney falls over in his chair, you can hear him say, "Hmm, a pretzel." His mouth doesn't move. (00:21:15)
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