Invincible Marine on Outskirts

In Halo 2 on the level Outskirts, when you play the level on Legendary; there is a marine that can become invincible. This can only happen when you are driving through the tunnel on a Warthog and you have a marine on the rear gun. When you get to the bit where the Covenant have positioned troops and ammo crates in the tunnel. If you drive through a gunfire of needlers and the marine on the turret is shot. From then on, he cannot be killed.