Deliberate mistake: Due to the thin tissue and blood supply to the lips, after Carly ripped the superglue from her lips she should have bled profusely, requiring steady pressure for a minimum of five minutes. However, that would have obviously slowed the tension of the movie too much.
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: When Amanda is watching the guy cut the skin from the back of his neck, you can see the same shot repeated twice at the beginning and end of the ordeal. It is the zoom in shot of her just as she is about to start shaking her head. (01:16:50)
Deliberate mistake: When Constantine comes out of the library after speaking with Gabriel, he stands beside Angela and none of them cast any noticeable shadows on the door and walls behind them. When it cuts to a shot above them, shadows are more being cast behind them. This is probably because of different light settings on the set. (00:23:35)
Deliberate mistake: When Dead Reckoning is firing the missiles at the end (towards the crowd that's being eaten by zombies), you can tell that they repeat the same shot of the missile firing three separate times, even though it's supposed to be three different missiles. (It's the shot of the exhaust coming out of the back of the launcher.) They try to mask it by cutting a little earlier or later each time... but it's obvious.
Deliberate mistake: Otis was portrayed as an albino in "House of 1000 Corpses" but in this film he has normal skin pigment.
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.