Three Johnsons glitch

The Three Johnsons Glitch is a simple glitch that can be performed on the Halo 3 Campaign levels Sierra 117 and Crow's Nest. The glitch causes multiple instances of Sergeant Johnson to spawn on the level.

Sierra 117
Start from the beginning of the level. Go to the waterfall and wait for Johnson. When he stops and looks at you, shoot him in the head. Quickly proceed to kill the enemies that deploy from the first Phantom, and get a loading point and/or checkpoint. Then, return to the first Johnson.

Melee and headshot him whenever necessary, until you reach the area where the second Johnson is captured. When you see the second Johnson fleeing from Covenant forces, headshot him with a Battle Rifle and use a Brute Shot and grenades to knock him off the bridge. Finally, melee both Johnsons to where the third Johnson (held captive by Brutes) resides.

Also, you can get two Johnsons at Rally Point Bravo. You will need two controllers to do this glitch. First, have the Master Chief go to the building where Johnson is held captive and go on top. There will be a Loyalist Phantom that will arrive shortly. When it is in range, shoot the gunner and jump into the Phantom and hold the RB button to operate the Plasma Cannon. When the Catch Skull is turned on, Grunts like to throw Plasma Grenades at the Phantom, which is now to be considered temporarily invincible. Oh, but Chief, you aren't. Have the Arbiter kill all enemies. After all this, he should go into the building and release Johnson. The Johnson you saw earlier will now spawn at the Pelican carrying an Assault Rifle. The second Johnson will be carrying the Needler. Now have the Master Chief get off the Phantom, and the Pelican's missile barrage will now do serious damage to the Phantom.

Crow's Nest
Start from the beginning to the level. Melee the first Johnson to the hangar; after you clear out the Covenant forces in the hangar, melee him back to the Ops Center, where you will find a second Johnson. Melee both Johnsons into the barracks -- the second, then the first.

When you reach the third Johnson (who should be about to board a Pelican), headshot him after all the nearby Marines have entered the Pelican. This will prevent Johnson, and also the Arbiter, from being removed from the level.

Now you have three Johnsons. However, the third Johnson and the Arbiter will vanish when you reach the room with the Plasma Cannons, leaving you with two Johnsons to melee to the end of the level.

Explanation
When proceeding through these levels normally, Sergeant Johnson is deleted from and re-added to the levels several times -- each time he and the player get separated, Johnson is removed from the level. Killing Johnson, however, prevents him from leaving the level. When the game re-adds him later, it does not check to see if the previous Johnson was successfully removed.

This, then, is the exact same glitch as the Life After Death glitch and the Two Arbiters Glitch (when the latter glitch is performed on The Ark or Halo).

Trivia

 * You can kill the Johnsons who spawn with a Battle Rifle anytime, turning them into an infinite supply of Battle Rifle ammo.