When Professor Gould runs up and tells Lisa that the test results are inconclusive, Apu's skin color changes to white (or rather, yellow as is usually animated for white) for one shot.
When Marge tells Lisa, "When that angel started to talk, you were squeezing my hand pretty hard.", this contradicts what was shown in said scene, when Lisa merely clung to Marge's dress.
When the mammoth tusk falls on Moe in the museum, he says, "I'm paralyzed! I just hope medical science can cure me." But his mouth doesn't come even close to matching what he's saying.
Agnes Skinner comes to the Simpson's house asking to rub her foot on the statue for luck for her upcoming foot surgery. A day or so later at the end of the episode Agnes is the first to sprint down the hill to the mall, unlikely for a person in need of foot surgery.