Stupidity: If Nick was desperate enough to have his door locks changed after Darian has been stalking him, then why does he leave his door wide open and allow her to come in when he discovers that Amy is in the hospital over the phone?
Stupidity: Amy goes into her darkroom and leaves the lock unlocked from the outside of the door so anyone (in this case Adrian) can easily lock her in. Most people going in that room would take the lock with them or lock the door from the inside to prevent this.
Stupidity: Why doesn't Nick just show Darian's parents or maybe even the police the 99 phone messages from her or the shrine underneath his living room? He had had enough of her at this point, and was even willing to move away at that point.
Suggested correction: If you watched the movie you should know that he did tell the police about the shrine. They checked and by that time Adrian removed everything because she's smart.
