Revealing mistake: When Chelsea is on the roof of the house and Kathy joins her, Kathy's hair is blowing in the wind, while Chelsea's stays perfectly still, the water in the lake is also perfectly still. Obviously the director had the actors do the scenes separately until we see them both together in which case both characters hair is blowing around.
Continuity mistake: At the end of the film, George falls off the roof and lands on the mud, getting his face and shirt extremely filthy. However a bit later, when the family escapes on the speedboat, he awakes and his face and shirt are now exceptionally clean. Admittedly the rain could have washed off his face, but not cleaned his shirt to that extent.
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.