Stiff Doors are thick versions of doors found in Robbery Bob 2: Double Trouble. They behave like normal doors but unlike normal doors, Bob cannot open them without sprinting into them (due to the stiff hinges), resulting in him crashing through them, alerting nearby Civilians or Guards.
Since they cannot be opened otherwise, Stiff Doors create a large CRASH!! popup on the screen to warn Bob about the noise it makes. Wearing the Black Hood costume will not nullify the sound, even if said costume is in full level.