NFS throwing errors in /var/log/messages

Jan 2 19:49:25 lab009 last message repeated 2 times
Jan 2 19:49:25 lab009 kernel: RPC call_verify: retry failed, exit EIO
Jan 2 19:49:25 lab009 kernel: call_verify: server accept status: 5
Jan 2 19:49:25 lab009 last message repeated 2 times
Jan 2 19:49:25 lab009 kernel: RPC call_verify: retry failed, exit EIO
Jan 2 19:49:25 lab009 kernel: call_verify: server accept status: 5
Jan 2 19:49:25 lab009 last message repeated 2 times
Jan 2 19:49:25 lab009 kernel: RPC call_verify: retry failed, exit EIO
Jan 2 19:49:25 lab009 kernel: call_verify: server accept status: 5

After restarting NFS the issue got solved.
But need to know why this occured.

Thx,
Siva.

I guess this has something to do with locked files over NFS, patches have been released in the last nfs versions.