Possible Duplicate:
Trouble downloading updates due to “Hash sum mismatch” error
Every time I try to update my system I get these messages:
W:Failed to fetch bzip2:/var/lib/apt/lists/partial/it.archive.ubuntu.com_ubuntu_dists_precise-updates_universe_binary-amd64_Packages Hash Sum mismatch
E:Some index files failed to download. They have been ignored, or old ones used instead.
I'm using Ubuntu 12.04 LTS. I don't understand what this message is about or what the problem is.
I solved using these commands: