Revealing mistakes: Near the beginning of the movie, some children are sliding on some "ice" which ripples under their feet - it's actually some sort of plastic sheeting.
Crew or equipment visible: The Ghost of Christmas Future takes scrooge to the cemetery near the end of the movie. At the beginning of this scene, when scrooge asks why they are here, you can see a rope at the bottom of the screen that pulls the Ghost across the graveyard to simulate it floating across on air.
Anachronisms: All the brass players in the bands are playing piston-valve instruments - about fifty years before they became available.
Anachronisms: The brass cornet and tuba in the beginning were not period pieces by a century or so.
Anachronisms: The spinet piano during Christmas Present was not available until the 1930's.
Revealing mistakes: The string holding up the Spirit of Christmas Past's bell is briefly visible when she removes her hand from above it in the first scene with Scrooge. The bell shakes and wobbles perceptibly (as the string is slightly moved) throughout the scene.