When running:

sudo apt-get update

I often get the following on a new 12.04 minimal install.

W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/main/source/Sources Hash Sum mismatch
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/restricted/source/Sources Hash Sum mismatch
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/universe/source/Sources Hash Sum mismatch
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/multiverse/source/Sources Hash Sum mismatch
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/main/binary-amd64/Packages Hash Sum mismatch
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/restricted/binary-amd64/Packages Hash Sum mismatch
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/universe/binary-amd64/Packages Hash Sum mismatch
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/multiverse/binary-amd64/Packages Hash Sum mismatch
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/main/binary-i386/Packages Hash Sum mismatch
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/restricted/binary-i386/Packages Hash Sum mismatch
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/universe/binary-i386/Packages Hash Sum mismatch
W: Failed to fetch http://us.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.

To fix:

Remove the content of /var/lib/apt/lists directory:

sudo rm -fR /var/lib/apt/lists/*

then run:

sudo apt-get update