Invincible Marines

Invincible Marines are, as you might expect, in-game Marines that have the peculiar trait of being completely invulnerable to all forms of damage. Such glitched Marines can be seen in all three games in the Halo Trilogy -- in one level of Halo: Combat Evolved, two levels on Halo 2, and two levels on Halo 3.

Pillar of Autumn
The Halo: Combat Evolved Invincible Marines can be seen on the level Pillar of Autumn. When Captain Jacob Keyes -- or any crewmember on the bridge -- is shot by the Master Chief, a group of invincible Marines come in and kill the Master Chief. This is not actually a glitch, though, but a programmed action for shooting the crew member and a fail safe to make the player die for betraying NPC allies. It has since been discovered that the Marines can be killed by using all of the player's frag grenades, but it is unlikely that a player could survive long enough to do this. The only other way to kill them is with mods, such as Devtrainer's Medusa or One-hit-kill.

Outskirts
On the level Outskirts, advance to the part where you are offered the Warthog after getting the hidden sword. The Marines inside will get in the gunner and passenger seat. Advance to the A Day At the Beach chapter. If you get out of the Warthog and attempt to attack the Marines, you'll find that they are invulnerable. These Marines might be useful when you have to go through the tunnel.

Metropolis
On the level Metropolis, play through until you get to the part with the two Marines in the Warthog at the tunnel. Destroy the Warthog before you get to the wall area with Jackals. The Marines will be alive, the Warthog done for, and your two Invincible Marines still alive with their invincibility intact.

The Ark
In The Ark, when you must activate the light bridge, kill some Marines so that they attack you on sight. In the next area, the Marine who drops off the Gauss Warthog will attack you on sight, and will be invincible.

The Covenant
On The Covenant, when two Marines land two Hornets in front of you, you must use one of them and fly to your right, where you will find a Pelican. Fly up to the Pelican's rear, and get out of your Hornet such that you end up inside of the Pelican. The Marine manning the Machine Gun Turret is invincible.

Trivia

 * Sergeant Johnson qualifies as an Invincible Marine in Halo 2 and Halo 3 because technically, he is one.
 * Commander Keyes is invincible in Halo 3, but she isn't a Marine.
 * The only place on the bridge where the Invincible Marines can not get you, is at the prow of the bridge, beneath the ladder.