"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 boarding 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.

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.