Invisible turret
From Halopedia, the Halo wiki
The Invisible Turret Glitch is a glitch present in Halo 2. It is an odd phenomenon involving the machine gun turret.
Sometimes, when a machine gun turret is destroyed, it will not respawn. However, the turret's stand will remain operable. Players can fire such destroyed turrets; the bullets will appear to come out of thin air. However, the turret itself (which isn't there) will not shield the operator from enemy fire.
On a side note, in Halo 3, you can destroy a Scorpion's coaxial machine gun, and it will still be fully operable.
