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.
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