When the answering machine is talking to the main character: They carry on a conversations and the answering machine is heckling him, you see the machine in every shot, then in the last shot from a distance, the machine is gone
I'd always thought of this break in continuity as intentional. Mike is carrying on a conversation with his answering machine, which couldn't really happen, which means it was all in his head. It seems too obvious to have been an accident.
Actually, every time the camera cuts back, something disappears off the desk. Eventually the answering machine is the only thing left, and then that disappears as well.