Continuity mistake: When Bart and the three other boys are standing by the wall after Bart sprayed a drawing of Principal Skinner, there are bushes by the end of the wall. But when we see Martin standing by the drawing, there are no bushes.
Continuity mistake: When Homer grabs hold of Bart's arm and demands to know what a "Kwyjibo" is, he is holding a brown skinned banana in his left hand. The banana remains in his hand when Bart is forced to explain what a "Kwyjibo" is, but when Homer begins to chase Bart, the banana has gone. (00:03:10)
Continuity mistake: When Homer picks up the yellow book from under the settee, you can see there is a white label on The Front cover. But when Lisa is reading the book, the label is now on the back cover, and when we see Lisa a few shots later, it is back on The Front again. (00:02:40)
Continuity mistake: When Marge is speaking to Homer during the Scrabble game at the beginning, you can see the tray in front of her is full of tiles. But when Homer takes away the book from under the settee Marge is sitting on, a piece has disappeared. (00:02:05)
Continuity mistake: When the teacher is looking out of the window, Bart has his paper in front of him on the desk. The paper disappears in the next shot, but reappears when Bart takes Martin's paper off the table and puts it on his desk. (00:06:10)
Continuity mistake: Millhouse's hair color changes from when we first see him to when Bart comes back to the school yard.
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