Unarmed Marine

The Unarmed Marine glitch is where the very last Marine in all Pelicans have no weapon but their hands are in the shape of a Rifle. When they get out of the pelican the rifle appears in their hands. Also if you manage to save the Scorpion Tank in Metropolis before it gets destroyed by the scarab, the Marine that is riding the tank is unarmed.

During the level Cairo Station, at the part where Johnson and Commander Miranda Keyes and two Marines are fighting off Covenant, you can actually take away a Marine's weapon. First, kill all the aliens. Then, when a Marine fires his SMG at a body swap weapons with him (it works best if you have a M6C Magnum. With luck, the Marine won't pick up the pistol and will run around without a weapon.