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, Arachnotrons, Cyberdemons, Demons, Spectres and Revenants can open doors thicker than 7 units. This is because they are faster than the other monsters (the faster a monster is, the thicker partly locked doors it can open).
Doors which are only locked from one side are rarely used by map authors, which is why the bug can only rarely be witnessed. The only instance of this type of door in the commercial IWADs is the red key door in Doom II MAP27.