Tree House of Horror XII - S13-E1
Continuity mistake: At the show the school puts on in "Wiz Kids", when the Montymort Dragon is first seen in the cage, the cage has 6 bars at The Front. When Lisa says "Release The Dragon", the cage has 4 bars. When the cage door is being raised, it has 6 bars again. (00:18:45)
Tales from the Public Domain - S13-E14
Continuity mistake: In the Troy scene, the letterbox outside the wall suddenly appears when the shot looks down to it. It wasn't there before and it isn't afterwards.
Continuity mistake: When Marge throws the Burly paper in the water, they start growing and sucking up the water. But the things inside that rolls don't grow.
Continuity mistake: When Homer first lifts the corpse up, it is wearing glasses. But when it cuts to the shot of the corpses' face, the glasses have gone.
Continuity mistake: When Homer opens up his bar there are 4 cars parked outside, but later when Moe opens the garage door there are no cars, but no one has left.
Tree House of Horror XII - S13-E1
Continuity mistake: In the classroom scene in the school Nelson is beating on Milhouse and leaves his head black, but when they go to get graded Milhouse is in front of Lisa (not Nelson) and his head is normal.
Tree House of Horror XII - S13-E1
Continuity mistake: In 'House of Whacks,' Homer swings an axe at the water softener, making three large dents. The shot changes and one of the dents near the bottom vanishes. (00:13:30)
Continuity mistake: When Bart is digging his hole, Homer goes over and starts talking to him, and you can see a spade standing up outside of Bart's hole. The spade wasn't there in the previous shot of Bart digging. (00:02:05)
Continuity mistake: When Homer gets out of the electric chair, the sponge has disappeared from on top of his head.
Continuity mistake: When Homer falls into the pool with green paint, most (but not all) of him is covered with paint. In the next close up shot, he's completely covered in paint.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Wiz Kids", when Milhouse is performing his trick at Springwart's Magic Recital, the trick goes wrong and he runs off naked, leaving his cloak on the floor. Later on in the scene, the cloak disappears.
Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when Homer attacks the tank, he puts the axe in the right hand side of the tank, but in the next shot the axe is now in the middle of it.
Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when the tools are attacking Homer, you can see one of the tools puts shaving foam on his face. But when he is walking down the stairs, the foam has gone.
Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when Homer is sitting on the settee, you can see his beer glass is empty, even though a few seconds ago it was nearly full.
Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when the dishes are cleaned, you can see by Marge's plate there is a spoon and a fork. In the next shot, there is now only a spoon, which disappears in the next shot.
Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when Marge has finished eating her meal, you can see a spoon on her plate. But when she is seen on the camera, the spoon has gone.
Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when Marge opens the door to the robotic salesman, you can see there is a green mat under the door. But when the salesman puts his foot between the door, the mat is missing, and in next shot the mat has re-appeared.
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.
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