Alone Again Natura-Diddly - S11-E14
Continuity mistake: In the episode where Maude dies, Homer puts Ned to bed in the right hand side. When Ned tells Rachel he's ready to smooth out Maude's shape, they smooth out the right hand side.
Alone Again Natura-Diddly - S11-E14
Continuity mistake: Homer's watch disappears when he is getting his car fixed.
Alone Again Natura-Diddly - S11-E14
Continuity mistake: When Homer is filming Ned cutting the bushes between the two front yards, the camera Homer is holding suddenly switches eyes.
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