Continuity mistake: When Dodger and Owen are talking outside the Halloween party, Dodger's cape is over her right shoulder. When Owen walks away after they finish talking, her cape is pulled back, off her shoulder.
Continuity mistake: In the scene where Owen uses the computer for the first time, we see that he's using instant messenger service. We see that he types "Ooh, I'm scared". However, later we see the message and now, instead of "Ooh", it reads "Oooh" (notice that now it has three "o", and before it had two). He couldn't change it, because the messages can not be edited when they're posted in an instant messenger service.
Continuity mistake: When the serial killer confronts Owen while he's cleaning the dining room, Owen's iPod just disappears.
Continuity mistake: In the scene where Owen and Tom pull Dodger from the pool. We see that Dodger's hair is hanging down over her face. We then see Owen's reaction, and then we return to Dodger. This time we see that her hair is behind her ear. She didn't have the time and it wouldn't make sense that she changed the position of her hair.