The courtesy reboot occurs when you are using your computer for a time-sensitive purpose, such as writing a report or making a presentation in a meeting. Without notice or even a BSOD, your screen blanks out and your computer reboots. On restart you frequently receive a cheery "update installed!" message, which makes your blood boil.
Presenter: Now let's take a look at the FY09 budget numbers, which we need submit in the next 20 minutes.
(Screen blanks out)
Attendee: Looks like you're going to have to wait for that courtesy reboot first, pal.