When resuming from hibernation, if Windows finds out that a USB hard disk has been removed while the computer was powered off it apparently decides that the best course of action is to bluescreen.
For added fun, if I then plug the hard disk back in and tell Windows to try again at restoring from hibernation, it bluescreens with a different error. If I then unplug the hard disk I still get the second error (which convinently blatted details of the first error from the event log).
Of course my PC then takes ages to start back up due to several years of accumulated crud. Most annoying.