Revealing mistake: The robber who kills Billy's mother is shown cutting her throat but with the flat side of the blade. Moreover real blood doesn't ooze out so thickly, indicating it was makeup.
Revealing mistake: During the nativity, when the baby is born, to the right of Mary, there is a boy tipping his candle on it's side. In the next shot, this same boy has the candle on his mouth.
Revealing mistake: When Richie wishes for the pie and instead gets a pig sty you can see the footprints of mud in the carpet as if crew and actors have previously walked through it. But it had just happened.
Revealing mistake: The width of the telephone pole behind Ebbie is about two-thirds the span of her shoulders but is just as wide when the background is blurred. (00:22:10 - 00:23:18)
Revealing mistake: Near the end, when Eliot enters Frank's office with his rifle, he accidentally shoots into the ceiling. He shoots it at a slight angle, but the rubble falls right on top of his head instead.
Revealing mistake: When the ghost (Bugs Bunny) tells Scrooge that he is going to take him to "the man in the red suit", the ghost puts his arms up but the shadow does not.