The rear view mirror in Dr. Orlof's car disappears and reappears throughout the movie.
The second time Martin enters Dr Orloff's office, they have a shot from behind Dr. Orloff with his elbows resting on the table and his hands to his chin. The very next shot, they are facing Dr Orloff, his hands are resting on the desk in front of him.
When Keith drives away in Martin's mom's moving van, Martin tries frantically to open the van door and smears the graffiti paint on it (indicating that it was still wet when they shot the scene or that it was meant to wash right off after filming).