Continuity mistake: After the murder with the yard sign, the killer puts the sign back into the yard, with no blood on it.
Visible crew/equipment: When one of the girls is killed in the kitchen and is out of the shot you see blood splatter on the wall. If you look at the bottom of the shot you can see the shadow of a hand tossing the blood at the wall.
Deliberate mistake: When the blonde girl is doing a strip tease, she has long hair which reaches far down her back. However when they show a flash shot of her breast as she spins, her hair is only to her neck. Obviously a double.