I've encountered some more errors under Ubuntu 12.04 LTS when issuing "sudo apt-get update":

W: Failed to fetch http://nl.archive.ubuntu.com/ubuntu/dists/precise/multiverse/source/Sources Hash Sum mismatch
W: Failed to fetch http://nl.archive.ubuntu.com/ubuntu/dists/precise/multiverse/binary-i386/Packages Hash Sum mismatch
E: Some index files failed to download. They have been ignored, or old ones used instead.

Easily fixed with:

apt-get clean
rm -rf /var/lib/apt/lists/*
rm -rf /var/lib/apt/lists/partial/*
apt-get clean
apt-get update
apt-get upgrade

Could be relevant for to other Debian-like Linux.

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.