When Stanley is taking a break and goes to sit down at the bar, a napkin suddenly appears during the cutaway.
Stanley takes Mr Carter to Room 625, they walk down a passageway and turn the corner. Room 626 is next to it on the right. When Stanley exits, Room 626 is not there. In fact he comes out of the last room on the left of the passageway.
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".
The camera's shadow is visible on several occasions.
When the staff helps the overweight Mrs. Hartung up the stairs you see the reflection of the boom mic raising up in the window
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.