After doing some apt-get update I got this error
GPG error: http://id.archive.ubuntu.com precise Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <email@example.com>
I had this since I add a new line to update my virtualbox to 5.1 in sources.list
But here is the way to correct the problem. I got this from here.
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 40976EAF437D05B5
It seem the signature key to update the ubuntu has change.