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:
rm -rf /var/lib/apt/lists/*
rm -rf /var/lib/apt/lists/partial/*
Could be relevant for to other Debian-like Linux.