So I've found 2 ways to softlock yourself. First if you fall in the lava pit there is no way of getting out plus lava deals no damage to the player. Have the lava kill the player as a doom-esque game usually does for example.
Second using a keycard removes them from your inventory, which means you wont be able to open the key door again. So say if you open red key door and see 2 enemies charging you, naturally you would back off and in the process softlocking yourself, because the red key door closes on you.