It doesn't seem to matter whether the soft or hard NFS mount option is used, it does not seem to matter that the connection to the NFS server is interrupted only temporarily. No amount of umount -f, umount -l, et al seem to help. This manifests itself even at shutdown. The system that has the stale file handle will refuse to shutdown as it is busy continually unmounting and failing to unmount.
*** This bug has been marked as a duplicate of bug 980088 ***