On the level "The Pillar of Autumn" on Easy difficulty, get to Captain Keyes as quickly as possible. When he hands you his pistol run out to the bridge and down the corridor where you load the pistol. If the glitch is done correctly, you shouldn't load the pistol, and the 3 Grunts shouldn't be at the end of the corridor.

Continue and pick up the Assault Rifle from the dead marine's body lying in the doorway then kill the Elites and Grunts and pick up a Plasma Pistol or Plasma Rifle. Go back to the corridor where you were supposed to load the pistol and keep back-tracking. Eventually, you should load the pistol and the 3 Grunts at the end of the corridor should appear. You will now have the pistol, the Assault Rifle and the Plasma Pistol/Rifle.