The War of the Simpsons - S2-E20
Continuity mistake: When Homer has caught General Sherman he beats it with an oar so that the oar snaps in two. Then suddenly Homer rows with both oars intact.
Three Men and a Comic Book - S2-E21
Continuity mistake: When Martin is tied to the chair near the end of the episode, the number of ropes tied to him change from four, then to three, and then back to four.
Three Men and a Comic Book - S2-E21
Continuity mistake: When Martin is tied up in the treehouse, at one point Bart takes the brick off the comic book and sets it down near Martin. When Bart and Millhouse fight, the brick disappears.
Three Men and a Comic Book - S2-E21
Continuity mistake: When Bart, Martin and Milhouse start to fight, the Radioactiveman issue 1 that they were protecting has disappeared from the table. Then when Marge brings up some food, they all start eating. The table has disappeared altogether.
Three Men and a Comic Book - S2-E21
Continuity mistake: When there is a rotating shot of Bart, Martin and Milhouse up in the treehouse, the shot never goes past the table, even though it should.
Blood Feud - S2-E22
Continuity mistake: When Mr. Burns gets up after the blood transfusion, Dr. Hibbert is incorrectly colored yellow instead of brown.
Continuity mistake: When Mr. Burns notices that Homer is wearing a pink shirt to work, when he pauses and zooms in the video, Homer is looking almost directly at the camera. In the next shot, though, he is looking more to his left. The people surrounding him also change. (00:03:05)
Continuity mistake: Bart opens the house's front door, then the shot zooms out to show us that all of Springfield is at the Simpson house. In the next shot with Mayor Quimby the door is suddenly closed. (00:15:30)
Continuity mistake: After "Michael" comes out of the Simpsons' car, there's a shot of the crowd's shocked reaction. Oddly, Groundskeeper Willie seems to have grey hair and brown skin.
Mr. Lisa Goes to Washington - S3-E2
Continuity mistake: When Lisa approaches the Abe Lincoln statue, there is an old person near her. In the next shot, there are three people suddenly surrounding her. (00:15:05)
Continuity mistake: At the beginning, when there is an overhead shot of the Simpsons' lawn, the broken lawn mower has disappeared. (00:01:20)
Continuity mistake: When Homer is on the sofa, the cushion under his head disappears when he wakes up. (00:03:00)
Continuity mistake: When Bart taunts Homer, he has nothing on his plate apart from his burger. When Homer runs over a second later there is more stuff on his plate. (00:03:30)
Continuity mistake: When Homer is smelling the BBQ the smoke that's in front of him vanishes instantly. (00:03:30)
Continuity mistake: When Homer is hanging out of the window there is a blue curtain tie visible however this disappears in the long shot. (00:03:30)
Continuity mistake: When Ned starts talking about the Leftorium during his barbecue, Marge's dress is pink in one shot. (00:04:05)
Continuity mistake: The colour of Bart's plate at the BBQ changes between shots. It goes from dark blue to light blue. (00:07:40)
Continuity mistake: When the family is having dinner, the glasses on the table keep disappearing/reappearing between shots. (00:09:30 - 00:10:30)
Continuity mistake: When Homer buys Ned's living room for $75, Bart is wearing a pink "Ned loves Maude" T-Shirt but by the end of the scene where Bart says "Hands off my china hutch." his shirt is the standard orange shirt. (00:14:40)
Continuity mistake: When Homer and Ned are talking after Homer tries to return the BBQ, the BBQ moves place on the lawn. Note the stepping stones. (00:17:40)
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