Simpsoncalifragilisticexpiala(Annoyed Grunt)cious - S8-E13
Revealing mistake: When Dr Hibbert is examining Marge's hairs (and I think another time in the episode), some holes are too low and you can see through her head.
The Old Man and the Lisa - S8-E21
Revealing mistake: At the hospital, while Lisa is explaining how much 10% of $120 million is to Homer, Dr. Hibbert's hand is yellow.
Revealing mistake: While Larry and Homer are talking about Mr. Burns, Larry's drink flickers light pink twice.
Revealing mistake: When Lenny is talking about Homer not needing a degree to work at the power plant, Lenny's five o'clock shadow briefly detaches from his face.