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: At the start, when Ellie rushes out of the ship's cabin she's wearing dark flat type shoes, but when Ellie climbs onto the deck railing she's wearing high heel sandals. (00:03:10)