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 Peter Pig and Donald Duck are dancing outside the "Idle Hour Club", you can see the door to the Club is open, but when the camera cuts to Donald, the door is shut, and in the next shot, the door is open again.