Deliberate mistake: In the beginning, the police chief says "The six members of the family were found last night, in bed, shot and killed." As he says this, look to the left of the screen. In the background, the real Amityville house (dark brown with white windows and trim) is shown instead of the house used in the rest of the film. That segment was from actual news footage of the murders from 1974; apparently, the director didn't want to recreate it.
Deliberate mistake: The mystery begins as Winston points out the postmark on the envelope that Dan receives, it is too faint to see where it's from. But whenever we see a close up of the envelope, there is no postmark at all.
Deliberate mistake: The chainsaw cuts off the German torturer's leg and it manages to move all by itself a few feet within a few seconds, when it would just run and slowly move and stall as seen after it moves. Obviously done to increase the tension. In addition, the chainsaw looks very clean with no blood dripping and bloodstains on the floor.