Continuity mistake: At the beginning, when there is an overhead shot of the Simpsons' lawn, the broken lawn mower has disappeared. (00:01:20)
Continuity mistake: When Homer buys Ned's living room for $75, Bart is wearing a pink "Ned loves Maude" T-Shirt but by the end of the scene where Bart says "Hands off my china hutch." his shirt is the standard orange shirt. (00:14:40)
Continuity mistake: When the family is having dinner, the glasses on the table keep disappearing/reappearing between shots. (00:09:30 - 00:10:30)
Continuity mistake: When Homer is smelling the BBQ the smoke that's in front of him vanishes instantly. (00:03:30)
Continuity mistake: The colour of Bart's plate at the BBQ changes between shots. It goes from dark blue to light blue. (00:07:40)
Continuity mistake: When Homer and Ned are talking after Homer tries to return the BBQ, the BBQ moves place on the lawn. Note the stepping stones. (00:17:40)
Continuity mistake: At the end Todd's shirt changes from white to red. (00:21:15)
Continuity mistake: When we see Maude ringing items up on the cash register, it has no external display. When Ned is ringing items up there is an external display. (00:20:45)
Continuity mistake: When Ned and Homer walk away from Ned's car, they are by the flower beds. Camera cuts and they are back by the car. (00:18:00)
Continuity mistake: When Ned starts talking about the Leftorium during his barbecue, Marge's dress is pink in one shot. (00:04:05)
Continuity mistake: When Bart taunts Homer, he has nothing on his plate apart from his burger. When Homer runs over a second later there is more stuff on his plate. (00:03:30)
Continuity mistake: When Homer is hanging out of the window there is a blue curtain tie visible however this disappears in the long shot. (00:03:30)
Continuity mistake: When Homer is on the sofa, the cushion under his head disappears when he wakes up. (00:03:00)
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