I almost had to reboot my computer today. I had my Mac Mini mounted as a smbfs and I did something dumb, I can't remember what. Probably rebooted the Mini or something. In any case Gentoo freaked out for a while, thinking the Mini was still mounted when it wasn't. Trying to umount it gave screwy "device is busy" errors.
One of the few things I've found that can force a reboot in Linux are deadlocked processes (I assume that's what it means; the state is listed as D). I'm not 100% sure but I assume this means deadlocked waiting for an I/O operation to finish. Even this doesn't really FORCE a reboot. It just leaves an unkillable process sitting around. You can ignore the process and keep working. But it bugs me in an obsessive-compulsive kind of way. In any case I was rather nervous that this might be the case today, but ps didn't list any processes in D state. I haven't really experienced a D-state process for a long while. Likely a year or so.
Anyways, for some reason, after 5 or 10 minutes the problem (whatever it was) went away and it let me umount the drive properly. It makes me happy when things work properly.