Revealing mistake: When the purser's safe falls through the ceiling, the ceiling metal is far too thin. Bearing in mind also, that the on the other side of the ceiling metal is supposed to be the floor of the next deck up, and that too is a thin flimsy metal, that doesn't even appear that it could support any kind of weight.
Continuity mistake: Before going to bed, Scrooge takes off his slippers and puts them by the side of his bed. But when he is disturbed by the singers, the slippers and the pillows he is resting his head on have disappeared.