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. (01:27:40 - 02:00:00)
Revealing mistake: Towards the end, when the family is getting away from the house in their boat, you can see that the motor is a covered camera. It is too high out of the water to be a real motor and when the shot is not from behind the boat you can easily see the "motor" moving left and right while the boat is still.