Continuity mistake: When Dave gives Homer the Bill Cosby book, the cover is light blue. When he reads the book in the garage, it is green. (00:09:40 - 00:11:30)
Continuity mistake: In the garage, Martin walks behind Lewis and the other kid. He jumps to walking in front of them in the next shot. (00:09:05)
Continuity mistake: In the video store, when Homer looks at sports tapes, the categories of movies behind him change from drama, horror, and 'Elvis' to drama, western and horror. (00:05:55)
Continuity mistake: Lisa asks Homer to go to Dr Hibbert's office, when she does, Homer's hand on the window changes places between shots. Note the vertical bars. (00:15:50)
Continuity mistake: When McBane's partner asks him to do just one thing for him, the picture keeps appearing and disappearing from his hands. (00:05:30)
Continuity mistake: In the garage Homer rolls up one sleeve but in the next shot both are rolled up. (00:10:35)
Revealing mistake: When the car arrives at the NFI, the car goes straight through the barrier. (00:09:20)
Other mistake: At the finish line, the crowd are missing all their appearance features such as faces, hair etc. (00:13:50)
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