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.