Factual error: A toy modern four-door automobile and a clear plastic ball can be seen as the children play when the Burgermeister comes in on his wheelchair. Both unlikely for a story set (presumably) in the 1800's, at the latest.
Revealing mistake: The wires holding up Jacob Marley's floating lock box and chains are visible. (00:32:10)
Revealing mistake: When Brewster is flying around in the Astro-Dome at the end of the movie, in a close up you can see the cables attached to him for a good second or so.