When Stanley is trying to call for the elevator and keeps missing the button the bell captain is giving him the evil eye. When the elevator door opens and Stanley falls in you can see Bob the bell captain break character and start laughing.
During the film's introduction sequence Jerry Lewis is seen writing his name using his finger on a white glass surface. When the camera zooms in it's clear the penmanship is different and was obviously written by two different people.
Several hand-lettered signs have obvious spelling errors. In the room where the bellboy steams pants, there is a notice about "tennants"; soon after, on the card warning non-guests not to watch the TV, there is a misplaced apostrophe in "yo'ure".