The Guardians

Summary
In Halo:CE, 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 by Death Barriers when trying to get out of a map, but it is not limited to that. Things such as glitches, scenery, objects and even teleporters can cause a death by the Guardians. The same goes for being killed in & out of maps by unmanned vehicles. If you wish to see this, the most simple way to see the message is in Halo 2 on the downloadable level Terminal. If you stand in front of the Train and let it crash into you, the message will pop up at the near the left corner of the screen.

Trivia

 * Getting killed by the Guardians has very rarely resulted in an unexplainable maniacal laugh being heard.