Monsters open locked doors

If only the front side of a door needs a key, the back side can be opened by monsters as normal. However, in certain cases involving thin doors, monsters may be able to open the door from the locked side (semi-locked). 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 semi-locked doors it can open).

The phenomenon 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 semi-locked 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 two yellow key doors in E2M2, and the yellow key door in the yellow key room in E2M6 of Doom, as well as the red key door in MAP27 of Doom II.

PWAD instances

 * Community Chest 3 intentionally exploits this bug in the mechanism of the secret exit in MAP15: Clash of Galaxies.

Trivia

 * In Wolfenstein 3D, any humanoid enemy could also open doors without needing a key. This would allow even for speedrunning, though can also end up trapping a player inside with no way out without cheats. It is marginally more justified here, as the vast majority of enemies the player encounters are guards on the level, and would thus have keys.