The Guardians

Summary
In Halo 2 and Halo 3 when a player in a multiplayer map or game is killed inexplicably (i.e. by a physics glitch, by the environment, or by an AI in Halo: Custom Edition), the message reads "(Player) was killed by The Guardians". This is sometimes triggered when trying to get out of a map, but it is not limited to that. Things such as scenery, objects, and even teleporters can cause a death by the "Guardians". The same goes for being killed in and out of maps by unmanned vehicles.

Basic Theory
It seems to be that the Guardians are a form of security, that is, they protect the map and the game from the players that attempt to (and also accidentally) go past the level boundaries or glitch the map in some way. This, if proven, would confirm a fact that shows that the normal map boundaries are just blocking off the outside edges, and preventing the player from violating the Sealed World Rules (a set of rules dictating level geometry and physics). Therefore, if the Guardians could somewhat be "glitched"; removed from active play, then these Rules could be exploited. However, the Guardians are well designed, in that there are very few manageable actions that a player can do short of hard hacking the game that will disable them.

Guardians should basically be assumed as a form of A.I. that shows that you were killed when the map stops the players from exiting the map as in Sandtrap and Snowbound (Halo 3), or when redesigned part of the map killed you such as the trains in Terminal (Halo 2) or any odd forms of deaths like falling Scorpions, solid objects such as crates hitting you at a high velocity, Teleporter glitches or any other deaths caused by unpredictable events. The Guardians can be overloaded. If the Guardians are in the form of a turret or mine but a secondary wall has been placed outside most maps to prevent players from getting too far. This wall cannot be "glitched" off.

Examples

 * When one of the Ghosts of Halo kills you, it will say, "Killed by Guardians".
 * On the Halo 2 downloadable level Terminal, if you stand in front of the train and let it crash into you, the Guardians message will pop up at the near the left corner of the screen.
 * In the Halo 2 multiplayer map Zanzibar, if you attempt to drive a vehicle up the windmill and you are successful your vehicle will explode and the message "[Player] was killed by the Guardians" will appear.
 * In the Halo 2 multiplayer map Waterworks if a player shoots down a stalactite on the ceiling and the player doesn't move and is killed by it the message "Killed by the Guardians" will appear.
 * In glitches where a Player falls through the bottom of map, it will sometimes state that death was by the Guardians.
 * In the Halo 3 map Sandtrap, if you travel too far towards the edge of the map, Brute Landmines launch from the ground and cause your death and the message "[Player] was killed by the Guardians" will appear.
 * On the map Snowbound if you overload the map not only will the shields disappear, but the auto-turrets will be disabled. You can do this on maps like Sandtrap, you can disable the mines and explore more of the area without being killed by the "Guardians".
 * In the Halo 3 map Narrows, if you fall onto the conveyor belt just below the bridge, you will die after an undetermined amount of time and a message will say "[Player] was killed by the Guardians".
 * If you manage to get out the map on Isolation and you go out too far, you will be killed by the "Guardians".
 * On Headlong there are gravity lifts that will sometimes glitch and kill you, saying you were "Killed by the Guardians".
 * In the Halo 2 multiplayer map Colossus if squashed by a container when it is falling, it will say "You were killed by the Guardians".
 * In Forge on Halo 3, if you make never-ending explosions, and try to get into them (without setting your damage tolerance to invincible via the Forge options menu), you will get killed by the Guardians".
 * In the Elephant glitch and the item falls, if it lands on you it will say "You were killed by the Guardians".
 * On forge, if the player creates a portal leading to the 'jackal room' on the map "The Pit", and walks too far behind the room, the player will automatically die, with the message "You were killed by the Guardians".

Trivia

 * Most equipment/power-up types, including Bubble Shield and Invincibility do not apply to the Guardians and you will continue to take damage as normal.
 * There have been many cases in both Halo 2 and 3 where mysterious deaths have been caused by the Guardians.
 * There is a guardian glitch in Rat's Nest.
 * Though unconfirmed, there could quite possibly be an easter egg, in Halo 3, in which you can hear evil laughter via your headset after killed by The Guardians. This was heard and accounted for by JayQs1919 when playing a custom game with a friend.