I have two machines, imaginatively called "one" and "nine" :) nine exported / to one, with no_root_squash. one mounted nine under directory nine. mount -t nfs nine:/ /nine nine was powered down. one's load went up to over 16, as users ran "df" occasionally. df never exited. I tried to unmount /nine as root. That process hung forever. I was forced to reboot the machine. It took quite a while for the nfs services on one to shut down.
This is not a bug, but typical behavior when an NFS server goes down.