Deliberate mistake: Towards the end of the movie, when Van Damme and the Sandman are fighting in the boiler room, the Sandman gets kicked into an open furnace for some time and then jumps out covered in flames. It's noticeable that he still has a full head of hair, which is impossible. Hair would be the first thing burnt right down to the scalp within a matter of seconds.
Deliberate mistake: In the whole of the 'Psycho' series, there has never been a closet door next to the dress cabinet in Mother's room, but for this movie there is suddenly a closet.