Instructions
Start a game in Forge mode with Forge World selected as your map. You need at least two players to use the invincibility glitch.
Press "Up" on the direction pad of one of the controllers. This causes the player using this controller to be in monitor mode. The other player has to stay in player mode.
Add two Armor Lock pickups using the monitor controller.
Pick up one of the Armor Lock pickups using the player controller.
Press "X" on the monitor controller while targeting the second Armor Lock pickup to bring up the item menu. Highlight "Delete All of this Object," but do not select it yet.
Use Armor Lock on the player controller.
Press "A" on the monitor controller to delete all of the Armor Lock pickups currently in Forge World, including the one used by the player. The player is now invincible, even though he no longer has or is using the Armor Lock ability.