4-Player Co-op Glitch

Introduction
In Halo 3 and Halo: Reach, this glitch allows 3-4 players to play locally on Campaign/Firefight with 4 screens.

Halo 3
In this game, this glitch allows 3 or 4 players to play Splitscreen Campaign. The effects of this is the same as playing 4-player Co-op Online, even the characters; 1st player being the Master Chief, 2nd player being The Arbiter, 3rd player being N'tho 'Sraom (The Sangheili wearing blue Combat Harness), and 4th player being Usze 'Taham (The Sangheili wearing dark crimson Assault Harness).

Side Effects
In most missions, the game will freeze if the player comes accross a Cortana HUD moment. This is because the game was only meant to load that part for only 2 players, not more.

Halo: Reach
In this game, this glitch allows 3 or 4 player to play Splitscreen Campaign. The effects of this is the same as playing 4-player Co-op Online, even the players' SPARTAN models.

Side Effects
In most missions (sometimes all missions), the game will lag or [briefly] freeze when their are a lot of character models loaded into the mission (UNSC troopers, Covenant troopers, etc.). This is because the game didn't load all of the needed resources to run a 3 or 4-Player Co-op Splitscreen Campaign, so it lags for a short time (sometimes, even longer with 4 players).

In Firefight, the effects are the same.

Advantages
This glitch allows the players to have a better advantage when fighting The Covenant. It also gives players additional reinforcements, like 1 or 2 extra Hornets in the Halo 3 mission, The Covenant, a Troop Transport Warthog (although overturned) on the Halo: Reach mission, ONI: Sword Base, etc.

Disadvantages
This glitches also gives the players a downside to their gameplay. Some of them are like experiencing lag and/or freezes in gaming, less visibilty (due to the 4 individual screens), player dying in the beginning of missions, etc.

Video Instruction:
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Text Instruction:
NOTE: This process takes a lot of practice (and timing), but it is worth it once you achieve the "prize". It will   also record your post-game reports if your profile is connected to XBox LIVE, so beware!
 * 1) Have 2 players sign-in and proceed to the Campaign or Firefight lobby.
 * 2) While in a Campaign mission or a Firfight game, press the "A Button" for the amount of players that are going to join.
 * 3) Quit the current mission/game.
 * 4) Start the game again, but this time press the "A Button" for the 3rd player a second after the final beep of the countdown. For 4 players, do the same thing, but (with near-perfect timing) press the "A Button" for the 4th player a split-second after the 3rd player presses "A". (Approximately just before the game says "Setting Up Game".)

Good Luck!
Good luck with playing with your friends, in real life!