Three weapons glitch

The Three Weapons Glitch is a weapons-related glitch in Halo: Combat Evolved. The glitch allows a player to carry three weapons at once in the Campaign level Pillar of Autumn. It is a useful technique on higher difficulties.

Instructions
When you finish talking to Captain Keyes, immediately turn around and go to the spot where you would normally receive ammo for your Pistol. If you reach the corner near the bulletin board before the loading point activates (before "Loading... done" appears), then you have started the glitch properly. The Grunts will not spawn, so you do not have to worry about being weaponless. Move forward, grab the Assault Rifle, and kill the Covenant forces in the mess hall. Pick up a second weapon.

Once all the nearby enemies are dead, go back to Captain Keyes. As soon as you're there, turn right back around and head back the way you came. This time, when you pass the loading point, you will receive Pistol ammo, giving you three weapons: an Assault Rifle, a Covenant weapon of your choosing, and a Pistol. You can switch between any of the three weapons at any time, and you can exchange any weapon for any other weapon (barring duplicates).

You can have any three of the following four weapons: Assault Rifle, Pistol, Plasma Pistol, and Plasma Rifle.