Plot hole: The hair that the dead girl has in her hand can't be from the guy who tried to rape her. They are too long to be his, and even if we accept that they are his, they are too curly, his hair is straight.
Continuity mistake: When Kevin is breaking into the window of his neighbour's house after the boy has shot himself, there is a statue of Mary that is there on the window sill, then the next shot it has fallen on the floor from the breaking window, and then the next shot it's back up again.
Other mistake: When he is talking to his sister in-law during work on the phone, the red wire, which was connecting them so they could talk, dropped or fell out of place. Even though this happened, the conversation still continued.
Character mistake: In the scene where Maggie tells her sister she is pregnant and how far along she is in her pregnancy, her sister says the child will be a Gemini, just like Einstein. Einstein's birthdate is March 14th, making him a Pisces. Lisa also mentions that Shirley Manson from the band Garbage is also a Gemini, when in fact she is a Virgo.
Continuity mistake: In the scene when Tom is digging in the backyard, he has no mud on his chest. Next shot cuts to Maggie talking to him, then cuts back to Tom, who now has mud on his chest. Next shot, back to Maggie and then back to Tom, and the mud is gone. This happens again in the next few shots between them with the mud appearing then disappearing.
Continuity mistake: When Samantha is pulling the boy's hair with her right hand, when he grabs her hand she is now pulling the hair with her left hand.