About once a month my AFS mounted file systems fail on my Mac. This is from one Mac to another, both running 10.8
[nelson:~] $ mount
/dev/disk2 on / (hfs, local, journaled)
devfs on /dev (devfs, local, nobrowse)
map -hosts on /net (autofs, nosuid, automounted, nobrowse)
map auto_home on /home (autofs, automounted, nobrowse)
//media@gvmini/media on /Volumes/media (afpfs, nodev, nosuid, mounted by nelson)
[nelson:~] $ sudo bash
[root:~] # umount /Volumes/media
Every process that’s touched that filesystem is marked uninterruptible. There’s a -f flag for forcing umount but it doesn’t work either, just hangs.
Hilariously, the man page for umount on the Mac says “Due to the complex and interwoven nature of Mac OS X, umount may fail often. It is recommended that diskutil(1) (as in, “diskutil unmount /mnt”) be used instead.” No, diskutil unmount doesn’t work either, even with a force.
At least the Finder doesn’t hang when you click the little eject arrow.
Update: naturally you can’t shut the machine down in this state either. Power switch for you!