Revealing mistake: When Darkman is hanging from the helicopter above the street, dodging traffic, in one shot from the side, the camera pans to follow a car, and you can make out the shadow of a crane. This makes it very obvious that in this shot, Darkman is in fact not hanging from the copter, but rather from the crane. (This shot has been cropped to remove the shadow in some subsequent releases, but it is still visible in the original Universal DVD release).
Revealing mistake: When Peyton looses his nerve and breaks the fingers of the game operator at the fair, you can see the fake hand that was used is a very cheap prop. It looks like hollow rubber the way the skin bends in and there is no sign of muscle/bone underneath.
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.