I was logged to two accounts when I started the 10.3.2 update. After it finished installing and was ready to reboot, I attempted to switch to the other user to log out and I got a message on the console saying that the CoreGraphics was incompatible. Additionally I could not start any new applications. I told the updater to reboot and it killed all my apps then just sat there. Subsequent requests to reboot were ignored.
Finally I ssh'ed in and killed the other user's loginwindow process. After that I was able to reboot but I lost the what I was working on in the other account. Be advised!
[robg adds: With the advent of fast user switching, this is a good general warning -- log out of all other user accounts prior to running a system update!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031218142002335