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 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 5 or 6 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 Devtrainer's Medusa or One-Hit-Kill.

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'll find that they are invulnerable. These Marines might be 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.

The Ark
Invincible Marines can be seen on the level The Ark. When you must activate the Light Bridge for the Scorpions, kill some Marines so that they attack you. In the next area, the Marine who drops off the Gauss Warthog will attack you on sight and will 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.

Trivia

 * Sergeant Johnson is an Invincible Marine in Halo 2 and Halo 3 because of him playing 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 KO'd 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.)