"Friendly" Brutes

The Friendly Brutes phenomenon is a glitch that causes some Brutes in Halo 3's Campaign to regard the player as an ally.

Crow's Nest
In Crow's Nest, where Miranda Keyes, Sergeant Johnson and the Arbiter board the last Pelican dropship leaving the base, set a Deployable Cover outside the door that the second wave of Jump Pack Brutes emerge from. Kill the Brutes that are present, and return to the Deployable Cover. Disable the shield, but do not destroy it, to enter the room. The Brutes present will no longer regard the player as an enemy, and although they will look at the player, they will not attack. If the Deployable Cover is destroyed, they will continue to exit the chamber and will regard the player as an enemy.



Tsavo Highway
This is also possible on Tsavo Highway, before the first tunnel. You must kill every other enemy in the area, including the Drones, and then enter the Deployable Lookout Tower near the cliff. The Brute that is up there will usually leave you alone. This only happens on Easy or Normal difficulty.

The Covenant
In the Covenant, if you hijack a Banshee, the Jiralhanae will not wield a weapon. Most of the time he has a Battle Rifle, or throws spike grenades, but rarely does anything, and stares at you. Sometimes he'll have normal damage, other times he's invincible, unless pushed off a cliff. Usually, their power armor will be removed, unless you're playing on a higher difficulty such as Legendary or the Mythic skull is turned on. Also, at Rally Point Bravo, if you go up any of the Deployable Lookout Towers and there is a Brute wielding the Brute Shot or the Fuel Rod Gun, the Brute will not attack you.

Explanation
The explanation is rather simple; the Brutes are simply coded not to attack anyone until they pass through the door they have been scripted to go through. This prevents, for example, the Jump Pack Brutes in Crow's Nest from shooting and throwing grenades from a position where they cannot be counterattacked, or a position where a single rebounding grenade in a confined space could kill every single one of them.

Trivia

 * This will work with other Brutes in other levels, and it may also work with other enemies.
 * One easy way to disarm Grunts is to turn on Mythic, Catch, Thunderstorm, and Tough Luck and start The Covenant at Mission Start on Easy and kill the Brute leader at the first tower to make the Grunts kamikaze. Allow a Grunt to collide with you; he should survive the explosion and when he recuperates, he should have no weapon.
 * For Jackals, allow them to overcharge their Plasma Pistol (they do this quite frequently on higher difficulties) and wait. When the overcharge disappears, it means that the Jackal is out of ammo.
 * On The Covenant, if you do this glitch, you can go upstairs in the first tower without activating the loading point and the Brutes will be unresponsive.