When the ambulance carrying Homer crashes, and Homer falls out the back, the gurney automatically unfolds, allowing Homer to fall back down the cliff. After Homer hits the bottom, and the gurney hits him in the head, the gurney is folded back up again.
When Bart shakes Dr Hibbert's hand in the hospital, the dressing on his forehead disappears.
When everyone is gathered at Springfield Gorge, awaiting Bart's arrival, Nelson says, "Where is he? I thought he said noon!" At the same time, Milhouse's mouth is moving, as if he is unconsciously mouthing the words he has heard in rehearsals. Throughout film history, this 'mistake' can be seen in inexperienced actors who have not learned to control this natural behavior. What's different here, of course, is that these are not human actors. The unavoidable conclusion is that this was an 'in-joke' intentionally included by the animators.