This is NOT a slip up - it is intentional! In the 70's and 80's, Detroit (but not the Japanese) tried gluing the rear-view mirror to the windshield. About six months after driving the car home from the dealer, the mirror would one day just fall off, exactly the way it does in the movie. I noticed this little "vignette" the very first time I saw Robocop in the theater! Robocop is just INCREDIBLE for this very sort of detail.