Continuity mistake: When Judge Harm is yelling at Marge and Homer after they have cut the rope, Marge is wearing lip gloss, and between two shots, it disappears.
Continuity mistake: While Bart is watching wrestling as Marge and Homer are struggling to do the dishes, the buzz cola can he is holding suddenly switches hands between two shots as Lisa is talking to him.
Continuity mistake: When Marge and Homer are put in stocks, Marge initially has lipstick on, but in subsequent courtroom scenes it has disappeared.
Continuity mistake: The stocks that are put on Marge and Homer are fitted so that their head and hands are secured and cannot be removed. Whilst in Flanders garage and Homer is using the rotary saw to remove Marge's stock, Homer's stock is still secured on him but his arms and shoulders are through the stocks.
Continuity mistake: When Homer is sleeping on Bart's school floor, Bart puts his shoe in Homer's mouth. Later, when Homer is screaming about "Cobras," the shoe has been coloured black, rather than the usual blue.
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