Tree House of Horror VII - S8-E1
Continuity mistake: In "The Genesis Tub", when Lisa takes the bowl of cola to the microscope on her dressing room table, you can see a brush near the edge of the table, but in the next shot the brush is behind the microscope.
Tree House of Horror VII - S8-E1
Continuity mistake: In "The Genesis Tub", when Bart comes into Lisa's bedroom with a balloon behind his ear, he goes to see Lisa. In the next shot, Bart is much near the door, and in the shot after that, he is near Lisa again.
Tree House of Horror VII - S8-E1
Continuity mistake: In "The Thing and I", when Homer puts a blanket on the floor for Hugo to rest his head on, you can see Homer is standing by Hugo. In the next shot, Homer is standing by the blanket.
Tree House of Horror VII - S8-E1
Continuity mistake: In "The Thing and I", when Hugo ties up Bart on the table in the attic, you can see that Bart's feet are tied up by pieces of rope. But when Dr Hibbert walks towards Hugo holding a picture frame, the rope is nowhere to be seen.
Tree House of Horror VII - S8-E1
Continuity mistake: In "The Thing and I", when the family are speaking to Dr Hibbert, there is a picture of Bart as a baby on the wall behind them. Later on in the scene, it disappears, and when Bart is by the staircase, it is back again.
Tree House of Horror VII - S8-E1
Continuity mistake: In "The Thing and I", when Homer and Marge return home in the pouring rain, Homer puts his hat on the hat stand. But when Bart is speaking to his mother, look at the hat stand and you can see the hat has gone.
Tree House of Horror VII - S8-E1
Continuity mistake: In "The Thing and I", when Bart is speaking to his mother after she has put down the phone, you can see Bart's arms are crossed, but when viewed from behind in the next shot, he now has his arms by his side.
Tree House of Horror VII - S8-E1
Continuity mistake: In "The Thing and I", when Bart, Lisa and Maggie are in the attic, Bart picks up a book and on The Front cover it shows a picture of Homer with the words "Homer, I Hardly Knew Me" by Homer Simpson. But when we see Bart with the book in the next shot, The Front cover is now completely clear.
Tree House of Horror VII - S8-E1
Continuity mistake: In "The Thing and I", when Lisa and Bart watch their father go to the fridge, you can see the spoons in their bowls are pointing to the left, but after a quick cut to Homer getting the fish out of the fridge, we cut back to Bart and Lisa and the spoons are now pointing to the right.
Tree House of Horror VII - S8-E1
Continuity mistake: In "The Thing and I", when the family are having their breakfast, you can there is a cup next to Marge's bowl, but when Homer gets up from his chair to go to the fridge, the cup is now near the edge of the table.
Tree House of Horror VII - S8-E1
Continuity mistake: In "The Thing and I", when Bart and Lisa come out of their bedrooms at the beginning, their doors are only partially open but in the next shot they are now fully open.
Continuity mistake: When the family are talking and Marge is sitting on the sofa, Bart runs and shoves Lisa (who is in front of Marge) out of the way, but in the next shot, she is still in front of Marge, despite the fact Bart pushed her out of his way.
Continuity mistake: When the Simpsons are driving to Cypress Creek the arrangement of luggage on the roof of the car changes depending on their stage in the journey.
Continuity mistake: When Homer is walking and talking with Hank Scorpio when he first arrives at work there is reversed "J" on Homer's tie that comes and goes between shots.
Continuity mistake: All of the cigarettes that Moe has smoked disappear from the floor after Marge has spoken to him.
Continuity mistake: The "Stinger" only appears on the boxing gloves after Moe passes them to Homer. It is not there in all preceding shots.
Continuity mistake: The extension cord Jimbo is using for a belt disappears after they run around the corner of the corridor whilst chasing Bart.
Continuity mistake: After Marge is looking at the matchbox from the Maison Derriere it just disappears when she sits on the sofa.
Continuity mistake: When the remote controlled plane crashes into the roof of the burlesque house, it is seen to hit only the side of the roof, yet when Bart goes to reclaim the plane, it is nested on the top of the roof.
Continuity mistake: At The Town meeting Mayor Quimby's tan disappears in the middle of the scene.
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