The Other Darrin: There were no less than three different voice actors for Arthur. And it was very noticeable. Two sound very similar: young and squeaky. While the other is the voice of a teenager who's gone through puberty. Two of which sound a lot like Mowgli or Christopher Robin. While neither of them voiced Mowgli or Christopher Robin, the older one did. Two of them are also animator/director Wolfgang Reitherman's sons.
What Could Have Been: The Wizard's Duel in the film, judging by concept art, was going to be very similar to the original book, ending with Mim turning into an Aulay (that is a giant equestrian beast with an elephant's trunk) just before Merlin turns into the germ.
Write Who You Know: The animators based Merlin on Disney himself, noting that they were both cantankerous, argumentative, but brilliant and ultimately lovable.