Factual error: Bart uses Milhouse's glasses as binoculars, but as Milhouse is near-sighted this wouldn't work.
Tree House of Horror VII - S8-E1
Continuity mistake: In 'The Thing and I' when Lisa and Bart are questioning Homer about the 'thing' up in the attic at breakfast, Homer is the only one eating. But in the next shot you can see Lisa and Bart with breakfast too. (00:01:30)
Tree House of Horror VII - S8-E1
Continuity mistake: In 'The Thing and I' when Homer and Marge return from driving around in the rain, there is no doorknocker on the door. Then in the shot where Hugo escapes, we see the door open with red scratches on it, and there is a doorknocker. Then later when everyone leaves to find Hugo, it's gone again. (00:02:40 - 00:04:55)
Tree House of Horror VII - S8-E1
Continuity mistake: In 'The Thing and I' Dr. Hibbert is about to explain Bart's irregular birth. In the closeup before the story, Dr. Hibbert has a stethoscope around his neck. This wasn't there in previous shots. (00:03:50)
Tree House of Horror VII - S8-E1
Continuity mistake: In "The Thing And I", Bart discovers he has a scar where he was separated from Hugo. During the attic scene, it disappears. (00:04:40 - 00:06:10)
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)
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)
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)
Tree House of Horror VII - S8-E1
Continuity mistake: In 'The Genesis Tub,' when Skinner is grading the universe in the tub, his clipboard changes from brown to blue. (00:17:55)
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.
Tree House of Horror VII - S8-E1
Continuity mistake: In Citizen Kang, when Homer bursts in at the Dole-Clinton Debate, you can see the wall behind him is light blue, but when we see a close-up of Homer, the wall is now brown.
Tree House of Horror VII - S8-E1
Continuity mistake: In "Citizen Kang", when the alien whips Marge's behind, you can see in the background there is a tree that changes to a crane in the next shot.
Tree House of Horror VII - S8-E1
Continuity mistake: In "Citizen Kang", when Homer pulls off the masks off the fake Clinton and Dole at the White House, the two transform into aliens and you can see Homer is between them. In the next shot, Homer is missing, and in the shot after that, he is back again.
Tree House of Horror VII - S8-E1
Continuity mistake: In "Citizen Kang", when Homer tries to save Clinton and Dole in the spaceship, Homer is standing behind a control panel and you can see there is nothing on left of it, but in the next a large lever suddenly appears.
Tree House of Horror VII - S8-E1
Continuity mistake: In Citizen Kang, when Kang and Kodos (as Clinton and Dole) are speaking to the man from the Democratic National Committee, you can see the building in front of them has a plant outside and there are shades on the windows. But when Homer appears in the next shot, the plant and shades have gone.
Tree House of Horror VII - S8-E1
Continuity mistake: In "Citizen Kang", when we see the family eating breakfast at the table, you can see that Bart has a bowl in front of him. But when Homer comes into the kitchen, there is now a glass by the bowl, and in the next shot the glass has gone again.
Tree House of Horror VII - S8-E1
Continuity mistake: In "Citizen Kang", when Kang and Kodos are transformed into Clinton and Dole, Homer is standing in front of the control panel and you can see in front of him there are several buttons, but in the next shot there is now only a green and red button in front of him.
Tree House of Horror VII - S8-E1
Continuity mistake: In "Citizen Kang", when Bill Clinton and Bob Dole are in the suspended animation tubes in the aliens' UFO, you can see that Clinton is facing left in one shot, but a few seconds later he is now facing right.
Tree House of Horror VII - S8-E1
Continuity mistake: In "Citizen Kang", when Homer is fishing, you can see behind him on his boat there are a brown box and a rolled up mat, which is standing up. But when we see a view of Homer's boat from the UFO, the mat is now lying flat on the back of the boat and the box is now coloured red.
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