The result of a remainder of an unbalanced equation inherent to the programming of Halo. It is the eventuality of an anomaly, which despite Bungie's sincerest efforts has been unable to eliminate from what is otherwise a harmony of mathematical precision. While it remains a burden to sedulously avoid it, it is not unexpected, and thus not beyond a measure of control. Which has led you, inexorably, to be killed by the guardians.