It seems like every week or so, OS X prompts me with some system updates that necessitate a reboot (usually Safari and iTunes - why the hell do I need to reboot my computer just because Safari was updated? This is the kind of behaviour people criticised Windows for in the past). I usually leave the dialogue box in the background for a couple of days until I'm ready to reboot, then go ahead.
Every once in a while (month), when I allow it to restart to "finish" updating, nothing happens. I start manually closing programs after getting warnings that each individual program failed to close (although manually closing them causes no problems at all). This time, the Software Updater seemed to hang as well, even though I had already told it to restart my Mac.
In the end, the machine seemed to shut down but the fan was still spinning. After hitting the power button a few times and waiting a minute or two, I held it to force a power-off and reboot. Then, instead of seeing the "Installing updates" dialogue you usually get when rebooting after installing updates, I just got my usual login screen, and sure enough, logging in as normal (after a ridiculous 4 minute delay before the Finder menu would respond to mouse clicks, WTF?) and opening the System Updater started the whole process from scratch.
LAME. Absolutely lame.