What does "nfssvc: Address already in use" mean?

Issue

The NFS service fails to start successfully:

# service nfs restart
Shutting down NFS mountd:                                  [  OK  ]
Shutting down NFS daemon:                                  [FAILED]
Shutting down NFS quotas:                                  [  OK  ]
Shutting down NFS services:                                [  OK  ]
Starting NFS services:                                     [  OK  ]
Starting NFS quotas:                                       [  OK  ]
Starting NFS daemon:                                       [FAILED]

Errors in the system log show it's due to the network port used by nfsd is current in use:

nfs: Starting NFS services:  succeeded
nfs: rpc.rquotad startup succeeded
nfsd[31070]: nfssvc_versbits: +2 +3 +4
kernel: nfsd: last server has exited
kernel: nfsd: unexporting all filesystems
nfsd[31070]: nfssvc: Address already in use
nfs: rpc.nfsd startup failed