Invincible Marines

Invincible Marines are certain Marines in the Halo Trilogy that have the notable trait of being invulnerable to all forms of damage. Such Marines that receive this trait through a Glitch can be seen in Halo 2 and Halo 3. Invincible Marines specifically programmed to be invincible can be found in Halo: Combat Evolved.

The Pillar of Autumn
The Invincible Marines can be seen on the level The Pillar of Autumn. When Captain Keyes or any Crewman on the bridge is shot (and likely killed) by the player, a group of around five or six Invincible Marines will enter and shoot the player until the player dies. This is a programmed action for shooting the crew and a fail safe to make the player die for betraying allies. It has since been discovered that the Marines can be killed by throwing all 4 of the player's Frag Grenades at them. Although it would be extremely difficult to survive long enough to do this. The only other way to kill the Invincible Marines is with Mods such as "God-mode" or such mods that would 'remove' the Marines from the level when fired upon.

Outskirts
Invincible Marines can be seen on the level Outskirts. Advance to the part where you are offered the Warthog after acquiring 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 will find that they are invulnerable. These Marines are useful when you have to go through the tunnel.

Metropolis
Invincible Marines can be seen 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 that has Jackals on it. The Marines will be alive but the Warthog destroyed. The two Invincible Marines will have their invincibility intact. Their invincibility appears to disappear when you reach the next section of tunnel after crossing the partially closed bulkhead; however, they remain exceptionally difficult to kill.

The Ark
Invincible Marines can be seen on the level The Ark. In the area following the activation of the Light Bridge, the Marine who drops off the Gauss Warthog will be invincible. However, when you go down to the Scorpion Tanks, he will no longer be invincible.

The Covenant
Invincible Marines can be seen on the level The Covenant.

When two Marines land their Hornets just before you encounter two Scarabs, 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 the Hornet so that you end up inside of the Pelican. The Marine manning the Turret is invincible.

Basically, all Marines manning the machine gun turret on a Pelican are invincible.

Trivia

 * Sergeant Johnson is an Invincible Marine in Halo 2 and Halo 3 because he plays a large role in the series and that his death before the final level in Halo 3 would result in the storyline being very inaccurate. In Halo 3 however, he can be knocked unconscious like the Arbiter. (They will fall as if dead but after a few moments they will stand back up and pick up whatever weapon that happens to be beside them.)