Revealing mistake: You can see Jackie's knuckles are already bloody BEFORE she starts pounding on the closet door.
Revealing mistake: When the lady is showing George and Kathy the kitchen, she casts two shadows because of the stage lights above her head.
Revealing mistake: When Matthew and Gregory go downstairs to answer the door, it's aunt Helena. Right before she pinches Matthew's cheek, you can see it's already red from a previous take.
Revealing mistake: At times throughout the movie there's a superimposed thunderstorm that rumbles or cracks every few seconds, purely for dramatic effect, and sounds exactly the same.
Answer: George Lutz.
How was it accomplished?