Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when the leprechaun knocks Marge's glass over on the table when she is eating her pancakes, the glass changes position between shots.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when Homer puts his spade in the ground when he is digging a hole with Bart, the spade keeps disappearing and re-appearing between shots.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when Lenny and Carl are crushed by the helicopter at the tavern, the liquid behind Lenny is white but changes to brown in the next shot.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when Homer, Bart, Lisa and Maggie are eating their breakfast at the table, they see Marge with a blue beard and Homer spits some food on the table. In the next shot, the food has gone, and when Homer goes to strangle Bart, it is back again.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when the Fortune Teller is speaking to Homer in her office, you can see the crystal ball is in front of her but in the next shot it has moved to the edge of the table.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when Marge, Bart and Lisa are at the Fortune Teller's office, you can see the Fortune Teller has three cards in front of her on the table. When Homer arrives, the cards have gone, and later on in the scene they are back on the table again.
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