Invisible turret
From Halopedia, the Halo wiki
This is a glitch that is only in Halo 2. In multiplayer and maybe in campaign, damage a heavy turret, both types will work. Make sure you do it with an explosion, or find a way to take it out in 1 hit. Half the time, if you walk up to it, it'll say "press and hold X to operate turret", even though there is no turret. And if you do hold X, you will be firing an invisible turret. If you leave it, you will still be able to operate it, as long as you don't go too far from the turret. This glitch gives no advantage, as there is no turret to block incoming fire directed at you.
In Halo 3, you can destroy the Scorpion Tank's coaxial machine gun, and it will still be fully operable. This was likely done to save time.
