Go to the room with the nuke you are supposed to find. Go down the ramp, then go under it. Crouch, then with your back to the bottom of the ramp, stand up. After a few tries, you should fall through the floor. You can kill enemies from down there, but you cannot get further in the mission. You must kill yourself or reset the game to get back up.
Mile High Club
Complete the game on any difficulty.
Stuck in Intervention mission
In the first part of the "Intervention" mission, get to where you go up the stairs to the room with one person inside and one on the balcony. When Col O'learc tries to knock out the terrorist at the computer, try to knock him out at the exact moment. It is possible to kill him. You must kill yourself in order to continue because he is the person who tells you what to do next.