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)
Other mistake: Bogeyland is just across the river outside of Toyland's gate, as we saw towards the end of the film, so it doesn't make sense that Barnaby had to climb down a well to enter it.