Revealing mistake: When Willy enters the apartment and heads for the stairs, Sam dives at him in anger and flies through him. In this shot you can see how poorly Willy was added into the scene. The coloration isn't even close and Willy looks 2-dimensional. (00:32:30)
Revealing mistake: When Sam is going back to Oda Mae's, when he's walking toward the door his reflection is visible walking the same way, though he's not in front of the window at an angle this would be possible for a few seconds. I don't know exactly what could cause on the filming side of the mistake, just that there's no way the reflection would look like that. (01:18:05)
Revealing mistake: When Carl is accessing the recently closed account, he moves to a second computer to attempt it. Look at the screen before he gets there. It already has the "account closed" and account information blinking on the screen. It's highly unlikely that he tried the account on that computer before, because when he first leaves his chair he looks around the room for an open computer. (01:31:30)
Revealing mistake: When Carl is being dragged away by the demons, you can see the cable line that he's attached to. May be fixed on DVD.
Revealing mistake: When the movie starts, Carl and Sam exit the subway station. A few members of the public/extras/background can't help but stare at the camera during the time it pans past them, firstly the man at the back as they are arriving up from the stairs, then the man with a hat, then the truck unloader on the right of the screen before they look at the red car. They all either seem to be starting at the camera, or looking or for thier cues to start acting.