Charging Needler glitch
From Halopedia, the Halo wiki
The charging Needler glitch is a glitch that works on the Halo 2 level, Sacred Icon or in Halo: Reach's Firefight mode. It can only be done when you have Jackals or Grunts (in Halo: Reach) as your allies.
To experience this glitch, you must play the Sacred Icon until you find the Jackal and Grunts, which are located at the beginning of the level.
Destroy all the Sentinels and the Sentinel Launchers to stop any Sentinels from coming out. If the Jackal is charging his Plasma Pistol after the battle, switch it with a Needler. If done right, the Needler will fire, but no Needles will come out. Try it with other weapons, such as the Okarda'phaa-pattern plasma rifle, or any other weapons you can find. If you give him the Plasma Rifle, he will fire and overheat it.
In Halo: Reach, start up a game of custom firefight, with at least one player playing as an elite (that is, on the blue team). When Jackals or Grunts are spawned during a wave, if one of them is overcharging a Plasma Pistol, the Elite player can swap the plasma pistol for a Needler. The AI will "overcharge" the needler, and though no shots are fired, the tip of the needler glows bright pink. A loud noise similar to that of a needler firing very quickly will continuously be heard a considerable distance away. This glitch can also be done with other weapons usable by Grunts, such as the Plasma Launcher (the Grunt will continuously charge up then fire the weapon), or the Fuel Rod Gun (one shot appears to be fired then nothing) and even Shade turrets.