Enemies in space glitch

On Cairo Station, after the first spacewalk, when you get to the ramp elevator go to the stairs to the right and jump down past the Drones to the bottom. After you and your buddies up top shoot a few of the Drones the airlock will start to open and, if you are quick, you can rush in before any Elites or Grunts come out and it will start to cycle through. Once it is finished the doors will open and you can continue, but the Grunts and Elites will still shoot at you through the airlock, even though the Elites have no breathing apparatus. The Covenant forces will not leave it unless you push them out (which you can do to the Grunts if you have patience, but not to the Elites) Once outside the airlock they will move around a little. This trick should only be tried on Normal or Easy. If you try on Heroic you will have to kill most of the Covenant before the airlock cycles through and on Legendary you will be lucky to make it into the airlock at all with the concentrated fire.

Also, after exiting the airlock if you get near enough to it and the grunts don't notice you, you can hear them making gasping noises as if they have been shot or are having trouble breathing.

An easier way to practice this is to proceed normally and kill all the Covenant except a Grunt. It helps if the Grunt panics and you start the elevator right away so the Marines don't kill the Grunt. Push the Grunt into the airlock. He will not shoot you inside the airlock, but he will out in space. This "pushing into space" trick may also work with Marines.