Continuity mistake: When Marge puts more lamb chops on Lisa's plate they are now grey in colour. Also the ones already on her plate have turned to grey.
Continuity mistake: When Homer is showing his BBQ invites to the kids Bart comes and stands next to Lisa and points to the extra 'B'. In the next shot, he's back over near Homer.
Continuity mistake: In the cafeteria scene, the shots from Lisa's perspective show glass protectors with signs where the food is. However, when the shot switches to Lunch lady Doris' perspective, the protectors are gone.
Continuity mistake: At the BBQ, Homer lights the grill on fire with a match. When Lisa walks out with the bowl of gazpacho, the fire instantly disappears. (00:13:05)
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