And for that matter, what ensures it is even correct the first time it is written?
(I think this is probably the far more common problem when I'm looking at a bug, newly discovered: the logic was broken on day 1, hasn't changed since; the comment, when there is one, is as wrong as the day it was written.)
Go ask Steve, he wrote it, oh, he left about 3 years ago... does anyone know what he was thinking?