Amnesia: The Bunker

Not missable. Shortly after you wake up in the bunker, you'll go through a mess hall and obtain the revolver (quit to the main menu after getting spooked when you've grabbed the ammo, and you'll unlock the Nope, c'est trop! achievement). Leave the mess hall afterwards and shoot the lock on the door to the left, go straight ahead into the room in front of you and it should pop while you're in there.