Revealing mistake: When we see a shot of Peyton typing (after he makes a synthetic glove for one hand), you can see the sides of either hand fade as they move about the keyboard typing. This reveals a "split-screen" effect with a fade through the center was used to match the real hand with the animated fake hand.
Continuity mistake: During the dancing freak scene, Peyton holds an iron bar and looks at his reflection on a puddle. He lets go of the bar, which falls on the floor. He then takes his hands to his head, but the reflection on the puddle shows his arms at waist level. Also, in the following shot the iron bar falls on the floor again.