Unsanity [via]:

When you see the “Optimizing System Performance” phase of a software update, Mac OS X is really updating prebinding. Updating prebinding has a very, very nasty bug in it. If multiple processes are updating prebinding at the same time, then it is possible for a system file to be completely zero’d out. […] Most of the real “random” failures reported on various Mac OS X “troubleshooting” sites after a user has installed an Apple software update are actually manifestations of this bug. […]

In order to prevent yourself from being smacked in the face by this bug, follow this simple rule: When “Optimize System Performance” appears during the update process do not touch your computer and definitely do not launch any applications. Just back away from your computer box as if it were a swarm of bees.

I always keep doing some stuff while updating my system; guess I’ve been lucky so far. Phew.