Trouble downloading packages list due to a "Hash sum mismatch" error

Just remove all the content of /var/lib/apt/lists directory:

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

then run:

sudo apt-get update

This is a known issue, and is exacerbated for clients behind proxy caches. Some large organisations and ISPs (especially in remote parts of the world) have transparent caches of which you may not be aware.

The fundamental issue is that the apt repository format is subject to race conditions when a mirror is updated. This problem particularly affects repositories that change rapidly, such as the development release.

You can track progress on the fix for this in this bug (please mark yourself as "affects me too" in the bug) and this blueprint. But be aware that it is a complex issue and may take more than one release to resolve.

The easiest way to fix this is:

sudo apt-get clean
sudo apt-get update