Three weapons glitch
From Halopedia, the Halo wiki
This trick allows the player to wield three weapons, but requires very good timing. To do this trick, play the level normally until you reach the ship's bridge. Let the cutscene unfold, then, after the Captain gives you the empty Pistol, run back outside as fast as you can. If timed right, the three Grunts that are usually outside will not spawn, and the player is free to grab the Assault Rifle that is normally in the door. After that, the player must pick up either a Plasma Pistol or Plasma Rifle, and then go back to the bridge and stay there until receiving a checkpoint. After that, proceed outside again and this time, the Grunts will spawn, and the player will receive the actual pistol (although the pistol will not instantly appear in the player's hands as it usually does, instead it appears when switching weapons). The player can then continue with the level and have three weapons. This trick can be very useful on higher difficulties.