D is for DOORWAYS
Random Pain-in-the-Ass Doorways of the Underworld
1 The door opens draw-bridge style, falling on whoever opens it for 1d12 damage and pinning them to the ground, unless they save vs paralysis to dodge out of the way.
2 Thick, oversized door has another smaller door right behind it, 1d12 smaller doors in fact, each new door revealed as the previous one opens.
3 The door revolves on a central axis instead of opening on a hinge. Each time a player character tries to go through, flip a coin. Heads they make it through, tails they're back where they started.
4 Door flies open violently at the slightest touch, throwing the first player character who attempts to open it 10' into the next room. They're unhurt, but spend 1 combat round lying prone.
5 Astonishingly transparent sliding glass door does 1d3 damage to anyone who walks into it (unless wearing a helmet.) Impossible to tell there's a door there at all until someone blunders into it, 1-in-6 chance of silently closing after each person passes through, super-slick surface frustratingly causes all attempts at marking the glass to slide off onto the floor.
6 Door loudly slams shut between each player character who uses it. It's no trouble to re-open it, but the noise means that there's a 1-in-6 chance that a wandering monster will appear in the emptier of the two rooms immediately after it closes.
7 Door cannot be opened by any means by player characters, but each exploration turn they spend attempting to open it, there's a 2-in-6 chance that a wandering monster will come through the door, opening it with ease and shutting it too quickly for the characters to keep it open.
8 Door was the entry-way to previous room occupant's lair. Previous room occupant was utterly paranoid, and door has 6 locking mechanisms. They all use the same key, but unless the player characters have it, they all have to be picked separately. All those locks also mean that it's twice as difficult as usual to batter down.
9 Door has a coin-slot and opens automatically after player characters insert 1d12€.
10 Largest available reasonably-intelligent wandering monster serving as a "bouncer" for the door, with an identical monster on the other side. It demands to see identification papers or hear "the password" before letting anyone through. It will also accept a bribe of 1d12€ per character, but would never be so gauche as to broach the subject itself. If the player characters attack, two more bouncers are nearby waiting for the shift change.
11 Door features large red button next to it. If player characters push the button, a bell dings, a light goes on above the door, soft music plays, and the door opens 1d6 exploration turns later. The door can be pried open without waiting, but reveals only empty black space, and anyone who passes through falls 50' before landing on back in the original room.
12 Door cannot be lock-picked or bashed down, but whenever player characters knock, instead of knocking sound, they hear a voice saying "Knock, knock." If they answer "Who's there?" and allow the door to finish the entire joke, it opens on its own.