Other mistake: After Pluto rises up from the Underworld, all the elves hide. When Pluto snatches Persephone and carries her away, the elves come out of hiding and begin to follow. The animation layer of the last elf on the left (turquoise hat) is not rendered properly, so his head and body appear out of thin air, mid-shot. (00:02:55)
Continuity mistake: When Stan and Oliver set the trunk on fire, the trunk flies up into the air and lands upside down. In the next shot, the trunk is now the right-side up.