When installing the the recent Safari 3.1 update, I had a 'heart-in-the-mouth' moment. I was viewing one of my other Macs using Leopard's Screen Sharing. With the remote Mac ready to install the update, I allowed Software Update to start the reboot process. By now, we've all seen how Leopard handles updates which install during a reboot: after the user has logged out, but before the reboot, the update installation is performed with a dialog showing the progress.

However, what I found out by accident when sharing the remote Mac's screen during this process was that the update pre-reboot progress dialog responds to the Command-Q key combination. In trying to close the Screen Sharing window, I accidentally sent my Command-Q to the update progress dialog, causing it to quit in mid-update. Installation of the Safari update was cut short and I was returned back to the login screen instead of seeing the Mac reboot.

Logging in (and even rebooting) confirmed that the update installation had been stopped early. In the case of the Safari update, many apps would crash on startup, with WebKit 'unresolved' errors showing in Console. In this instance, the system was still usable enough to copy across the Safari update package and reinstall it manually to fix things. However, hitting this problem during deeper updates -- such as a 10.5.x release -- could leave an unbootable system.

The upshot is, when the pre-reboot update progress window is showing, keep your fingers away from the Command and Q keys, and be particularly careful when performing the update within a Screen Sharing session.

[robg adds: I haven't tested this one, but if true, I hope that a future Apple update will block the use of Command-Q at this stage of an update (or at least provide a confirmation dialog and a safe recovery after a reboot).]