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, 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).

This occurs when the monster "bumps" in the door fast enough that part of its collision box reaches the other side of the door, which is unlocked, and thus opens it. This is caused by the same glitch that allows grabbing items behind a wall.

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 instances of this type of door in the commercial IWADs are the red key door in Doom II MAP27, the yellow key door in the yellow key room in Doom E2M6 and both of the yellow key doors in Doom E2M2. Community Chest 3 MAP15: Clash of Galaxies intentionally exploits this bug in the mechanism of its secret exit.