Recently I've encountered an error when issuing "sudo apt-get update" on Ubuntu 14.04 LTS:

W: GPG error: http://download.webmin.com sarge Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D97A3AE911F63C51

An easy fix which worked for me was:

sudo gpg --keyserver pgpkeys.mit.edu --recv-key D97A3AE911F63C51
sudo gpg -a --export D97A3AE911F63C51 | sudo apt-key add -

The problem and fix could apply to other Debian-like Linux.