When updating your Mac, don't touch nothing
I just did some updates on Macs running OS X 10.4 and 10.3, and while I'm too cautious to do anything that would possibly interfere with an update while it's downloading and installing, others aren't so cautions, according to unsanity.org, via Infinite Loop:
Every single time you install an update to Mac OS X whether it be an iTunes update, a QuickTime update, an update for daylight saving time, a security update, an Airport update, or an actual Mac OS X update, you can be hit by 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. Yes, it does mean that if you install the Mac OS X 10.4.9 update, you may get hit by the bug.
Of course you are doing a full backup to a Firewire-equipped drive before you install your updates, aren't you?