When Michael sneaks into the basement, pay attention to the scene with the cell phone. Cell phone breaks before hitting the ground and then the cell phone magically appears back on the deep freeze in the next scene.
When Michael gets out of the shower, he towels off his head which dries his hair and fluffs it out in all directions. Then he immediately wraps the towel around himself and goes for the door. When he opens the door, his hair is suddenly slicked down in perfect style and dripping water on his face.
When Micheal and Kelly were looking through Davids items in his room Micheal and Kelly have to hide due to David arriving home, when David is looking for Micheal and Kelly he enters Michaels room, near the door on the wall there is a unframed poster of a model, later in the movie when David is talking to Micheal in his room the poster is not there any more and a small framed photo is leaning against the wall were the poster should have been.
In the beginning David Harris cuts himself and places a tissue on the left side of his chin. As he leaves the bathroom and turns around for the camera shot, the tissue is on the right side of his chin. This is because the image is a mirror shot, so it appears that the tissue is on the wrong side.
At the end of the film, David, now Adam, could not possibly expect his next ruse to work. After all, he left everyone else alive, sans Jay, Mrs. Cutter, and Susan's friend, being they were all about to talk to the police. Jay found out he never went to the University Of Oregon, Mrs. Cutter recognized him from "America's Most Wanted", and Susan's friend was trying to get Jay to do a background check on David. With the remaining family left alive, they could all call "America's Most Wanted" and say that the man living with them was the same man wanted for the previous killing. David, now Adam, does not don a very convincing disguise at the end being his face is the same, he has practically the same hair color, and is still clean shaven. The only difference is he now has longer sideburns. Isn't it possible someone else will recognize him wherever he is now?
When the ex-husband arrives at the family home to say goodbye to his kids, he has a taxi waiting outside. The ex is then murdered, and waiting taxi disappears from the plot, whereas you would have expected the cabbie to come to the door to ask where his passenger is?
There is a large storm that the son and his girlfriend are seen sitting in the car and he finally runs into the house and she is left in the car watching him enter. In the background of the neighbors circular driveway you can see where the "rain" has a nice arc on the driveway and the area farther away from this arc is dry.
There is a scene where the mother comes out the front door and part of the interior of the house is seen. The staircase for the house is on the left side of the door; however, in other scenes from the movie the staircase is shown as being on the right side after entering the house.
The goof item below may give away important plot points.
After David drowns Susan's friend in the pool (i.e., after she is dead), the positions of her hands change back and forth between shots. Shots above show both of her hands pushing against the edge of the pool, shots below the water show the hands not touching the sides of the pool.