"Turret Detach Glitch" refers to two separate glitches in Halo 3. The first glitch duplicates a Machine Gun Turret; the glitch is separate from the Forge Turret Glitch despite its identical result. The second glitch involves activating a loading point while a turret is detached.
While trying to detach a turret, let go. There should be a turret in your hands and one still on its original place. Activating the Indestructible Vehicles in Custom Games also allows for such duplication, by simply breaking and re-manning the turret repeatedly; this method is more an exploit than a glitch, however.
Loading Point Edit
When playing the Campaign on two-player split-screen Co-op, if one player (Person A) crosses a loading point at exactly the same time that the other player (Person B) detaches a turret, then Person B will spawn and lose control of their character. Both screens will show Person A's view separated by a random patch of black and green. Person B's screen will show their HUD with a permanent "Press B to detach turret" message and a turret Reticle. In this state, it is possible to walk through walls and see outside the map.