Tree House of Horror VII - S8-E1
Continuity mistake: In 'The Genesis Tub', in between Bart taking the tub and the school science fair, Lisa's slippers disappear from her feet.
Tree House of Horror VII - S8-E1
Continuity mistake: In "The Thing and I": After the kids have found Hugo in the attic, when Marge and Homer return home, both front door knobs (inside and outside) do not have backplates, but once Hugo escapes both door knobs have backplates. The two backplates vanish and reappear repeatedly in following shots. It's funny because when they leave to search for Hugo, Homer shuts the door with no backplates, but then Bart uses a key in the backplate to lock The Front door.
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.
Tree House of Horror VII - S8-E1
Continuity mistake: In 'The Genesis Tub,' Lisa sits on a tooth-throne in the tub, which has red cloth draped over the arms with gold tassels around the edge. However when Bart comes to take the tub, Lisa stands up on the throne and yells "Bart, no!" In this shot the tassels are gone. (00:17:45)
Continuity mistake: At The Town meeting Mayor Quimby's tan disappears in the middle of the scene.
Continuity mistake: When Mr Burns and Homer first enter the cabin, you can see the door opens with a door knob which is on the right hand side (looking from the outside). Later, after the avalanche, the door seems to open a different way, the knob has been replaced by a handle and it's on the other side.
Simpsons Spin-off Showcase - S8-E24
Audio problem: In Moe's spinoff, when we first hear Grampa's voice, you can see the love tester in the background but the love meter lights do not light up when he speaks.
The Old Man and the Lisa - S8-E21
Continuity mistake: Right at the start when Lisa says "I could still use more paper," the plastic can-rings she was holding vanish in the next shot.
Tree House of Horror VII - S8-E1
Continuity mistake: In 'The Thing and I', at the end everyone is having dinner. Dr. Hibbert says "Care for a drumstick Hugo" and everyone's plate is empty. Seconds later everyone has food without it being served out. (00:06:50)
Continuity mistake: John is wearing Homer's old "Pin-pals" bowling shirt when the Simpsons first meet him, but only after we see the back of the shirt does the name "Homer" appear over John's breast-pocket; clearly there was nothing there during their entire previous conversation. And even if this is a deliberate "mistake" for comic purposes, in 7th season episode"Team Homer", the very same bowling shirt is ripped to shreds when Homer is being mauled by Mr.Burns' dogs at the very end.
Continuity mistake: When Marge arrives after the song, Flanders and Lovejoy are talking to her, and behind them you can see a crowd of people. The crowd then disappears in one shot, only to reappear a shot later.
Lisa's Date with Density - S8-E7
Continuity mistake: When Jimbo and Co are 'coleslawing' Skinners house, he wakes up when coleslaw hits his window. When the shot moves to outside, there isn't any coleslaw on any of the windows.
Tree House of Horror VII - S8-E1
Continuity mistake: In 'The Genesis Tub', Principal Skinner takes the lid off of the tub, and puts it on the table. In the next shot it's gone. (00:17:55)
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: All of the cigarettes that Moe has smoked disappear from the floor after Marge has spoken to him.
Continuity mistake: At the end of the episode when Marge is first seen doing the entertainment, the shot shows (from left to right): Flanders, Skinner, Kirk Van Houten, Kent Brockman, Mayor Quimby, Grandpa, and behind them, Lionel Hutz, Lenny, Homer and Dr Hibbert. In a following shot from behind Marge the guests have changed places. Reverend Lovejoy is on the end, then Flanders, then Skinner, then Mr Burns, then Grandpa. There is a seat next to Grandpa which is blocked by Marge's head. Also Lionel Hutz has moved far back. Where did Burns appear from and where did Kirk, Mayor Quimby, and Kent Brockman all go?
Continuity mistake: In the cafeteria at the power plant, in the wide shot the clock on the wall is on 9.00, but closer up it's showing 10.35.
The Springfield Files - S8-E10
Continuity mistake: After the scene with the T-shirts, when the alien arives, Marge's "Homer is a dope" shirt disappears and she has her normal dress on.
Continuity mistake: When Bart falls down the stairs, you see him fall down at least 50 stairs in the close up. There are never this many on the full shots of the staircase.
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