Rapid firing Fuel Rod Shade
From Halopedia, the Halo wiki
The glitch is triggered when an enemy overcharging its Plasma Pistol climbs inside the Fuel Rod Shade to man it. While a Fuel Rod Shade can normally fire only a few shots at a time before needing to cool down, said cool down time will be lost and the turret will be able to fire infinitely and instantaneously. The shots appear to "overcharge" when the player is not visible to the gunner, but once the player is in range the shots fire in a continuous beam akin to that of a Scarab Gun. This may prove to be a massive hindrance to the player, who will have incredible difficulty trying to eliminate the gunner without being killed. The best option is simply to restart at a previous checkpoint, when the gunner had not yet entered the turret.
The glitch is possible on any level a Fuel Rod Shade is located at, such as the very beginning of the level Tip of the Spear. The gunners controlling it will always be an Unggoy, as Kig-Yar and Yanme'e are capable of overcharging their Plasma Pistols but can not man Shades, and typically higher ranked Unggoy, who tend to overcharge their Plasma Pistols more often.