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: Marie and Nutcracker go down the river in a swan shaped boat, and Nutcracker is standing up on the back moving the boat along with large pole. In the water, you can see the reflection of the boat and Marie the way they are supposed to be. But the reflection of Nutcracker is not moving, and the pole is not reflected.