Continuity mistake: After Bart attempts to escape by using spray paint cans as rockets, his feet alternate between clean and spray-painted for a few shots.
Continuity mistake: Before Bart notices the lemon behind the lemon-shaped rock there is actually no lemon there. We can later see that there is too much of a significant gap between these two objects for the lemon to be hiding behind the rock.
Continuity mistake: When Milhouse is spying with his binoculars with an camouflage outfit on, the colours of some patches change in some other shots. E.g The bottom left corner (from behind) changes from dark green to light green.
Continuity mistake: When all the dads are together in front of Homer's house, one of them is wearing a grey sweater. When they enter the RV the man with the grey sweater doesn't, but throughout the episode he keeps appearing and disappearing on the RV.
Continuity mistake: When the Shelbyville kids drop the basket of lemons when Bart and friends start throwing lemons at them, the position of the basket changes when the Shelbyville kids run off. Note the colour of the Springfield grass and the Shelbyville grass.
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