In Halo 2, the Guardians are an excuse for a glitchy death, say when you try to superbounce and you fall through the ground.
In Halo 3, if you were to try and get out of a map, the "Guardians" would kill you.
So, Guardians are pretty much an excuse for your death while trying to do something that your not supposed to do.
Examples:
In the Halo 3 Beta, if you were to try and BxR, you would explode. Killed by the Guardians.
In Halo 2, like I stated above, if you try to superbounce but fall through the ground, you were killed by the Guardians.
if your playing snow bound and you go past those turrets they will kill you an it will say "you were killed by the gaurdians"