In the Intervention mission, you can fall through the floor by doing the following. Go to the room with the nuke that you are supposed to find, then go down the ramp and then under it. Crouch, then with your back to the bottom of the ramp, stand up. You should fall through the floor. You can kill enemies from underneath, but you will be stuck and cannot advance in the mission. You must kill yourself or reset the game to get back up.