This morning, I began building a new user account for my demos tomorrow at NFJS. I'm doing this because the last time I did demos was a disaster. Since then, I've rebuilt my hard drive, and while things seem faster - a clean user account is the best way to get maximum efficiency. While building the new user, I was prompted by Software Update, and promptly installed Backup 3.0.1 and OS X 10.4.3.
After configuring a number of settings, I rebooted the machine. Now all I get is a gray screen with a dark gray apple on it. The damn thing won't boot. I rebuilt the disk with DiskWarrior, but no dice. Now I'm trying to repair permissions and all that jazz with Disk Utility.
Luckily, I can still boot off my iPod and I can always rebuild an external Firewire drive if I need to. However, I'd much rather get back to where I was this morning. There's nothing like hosing your machine the day before you really need it.
Update: I fixed the problem by reinstalling OS X with the "Archive and Reinstall" option. No data loss, time to backup.