If you turn on the new Filevault full-disk encryption on a system that has more than one user, and have a secondary user (possibly limited to ones not authorized to unlock the disk), then if you allow the screensaver to activate (which by default will also lock the screen), clicking the “Switch User” button can cause a kernel panic. Not all screensavers will do this, but it is 100% repeatable for the ones that do.

Ironically, I first had this happen while running Jamie Zawinski’s BSOD screensaver…

[Update: okay, switching to a “safe” screensaver isn’t good enough if you actually log into the secondary account and then log back out. That’s triggering a panic for me, even with one of Apple’s supplied screensavers. Also, once the disk has been unlocked by an authorized user, it appears non-authorized users can log in just fine.]