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.

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.