Sometime you get signature verification error when you run apt-get update. Bellow is a sample output:

W: GPG error: http://ppa.launchpad.net lucid Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 83FBA1751378B444

To resolve the issue run the following command:

root@fakrul3350-laptop:/home# sudo apt-key adv —keyserver keyserver.ubuntu.com —recv-keys KEYID