Revealing mistake: Anytime the camera is on Richard Dreyfuss' hands while he's playing the piano, it is clear that he is not actually playing; his hands are frequently in the wrong part of the keyboard for the sound being produced. At other times, when his hands are moving down the keyboard, the pitch we hear is going up, or vice-versa.
Revealing mistake: Their mother tells Fritz to come with her out of the room so Drosselmeier can visit with Marie. As he is getting up, a sudden imperfection in the art appears as a crude white line appears on the chest by he bed and stays there as he moves.