Monsters open locked doors

If only the front side of a door needs a key and the door can also be opened from the back side without a key, a monster can sometimes open the door from the locked side. All monsters can do this, but only Arch-Viles, Cyberdemons, Demons, Spectres and Revenants consistently open locked doors. This is commonly misconceived as an engine bug. This only happens when the back side of the door isn't given the lock as well, and therefore is a mapping bug.