The Guardians

Summary
In Halo: Combat Evolved, Halo 2 and Halo 3 when a player in multiplayer 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 obviously 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 hardhacking 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 predesigned 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.

Examples
if you turn into the moniter in sandtrap and go high enough the mines will not be set off.
 * 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.
 * The turrets in the Halo 3 map Snowbound count as Guardians if the player is killed by them.
 * 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, mines launch from the ground and cause your death and the message "[Player] was killed by the Guardians" will appear. Interestingly, these mines appear to be a design similar to the Spike Grenade. This may be because Brutes inhabited this area and planted mines to protect themselves.
 * On the map Snowbound if you overload the map not only will the shields disappear, but the autoturrets 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 conveyer belt just below the bridge, you will die at an undetermined amount of time and a message will say "[Player] was killed by the Guardians".

Trivia

 * Getting killed by the Guardians has very rarely resulted in an unexplainable maniacal laugh being heard.
 * The unexplainable maniacal laugh is actually an Elite taunting the Player after dying.
 * Most equipment/power-up types, including Bubble Shield and Invincibility do not apply to the Guardians and they will continue to take damage as normal.