The front door on the Doyles' house as filmed from the outside of the house is not the same door as it is as filmed from the inside. As shot from the outside of the house, the doorknob is on the right side with the hinge side on the left. If this was the same door the inside would be the opposite-- as you face it the doorknob would be on the left and the hinge side on the right. But it's not --as shot from the inside the doorknob is on the right and the hinge side is on the left. You can see this slip-up twice. The first time is when Laurie leaves the house after the kids have gone to bed to investigate why no one is answering the phone across the street. The second time is when Laurie is trying to get back into the house as she's getting chased by Michael. She can't get the keys out of her pocket so she throws a potted plant up at the window and wakes Tommy up and he opens the door. Using a different interior of a house or building is common practice in film making for obvious reasons: aesthetic, logistical, etc., but this slip-up is a classic example of the film makers doing a crappy job of matching the interior to the exterior.