The first sign that something was off was that the App Store indicated there were 3 updates, but displayed only 1 (the 10.8.3) update. (I had already installed it successfully on my 2012 MacBook Air, where this bug did not appear.) The bug appeared on my 2012 Mac Mini running 10.8.2. I thought, well, since it went OK on the Air, it'll go OK on the Mac Mini.
I downloaded the update, and it seemed to install at first, but the progress bar for the installation seemed to hang, saying "4 minutes remaining." I left it like that for an hour, but then rebooted by pressing and holding the power button.
The machine booted normally, much to my surprise. It shows 10.8.2 and Safari 6.02, so I'm not if anything was actually updated before the hang--I haven't (yet) noticed any app crashes, etc.
Interestingly, now the App Store shows all 3 updates I had available (10.8.3, Xcode, and PST Bridge).
I'm wondering if there are any ways to detect if the update was partially installed? I'm now extremely hesitant to try the update again since I (seemingly) made it through a failed OS update unscathed.