Continuity mistake: In the beginning, the family is eating in front of the T.V. When Marge is complaining about getting the food all over the floor, she drops her meatball on the floor, and it leaves a trail of sauce on the rug. In the bird's eye view of the room, the sauce has disappeared until the shot changes: then it reappears.
Continuity mistake: Marge said to Bart she would make Butterscotch chicken for him, but in the Season 2 episode "Blood Feud" Marge says Bart's allergic to butterscotch.
Continuity mistake: While Marge and Homer are standing by their front door when everyone leaves the house Marge is wearing earrings, but when it cuts to the closeup both of Marge's earrings have vanished. (00:08:10)
Continuity mistake: After Marge has bitten the candle down to size, when she wipes Bart's face her left earring vanishes. (00:03:35)
Continuity mistake: After Marge bites the candle down to a smaller size, the candle turns back to its original size for a frame.
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