Bug#499379: Hash sum mismatch
I get similar errors, for instance with apt 0.7.20.2 on amd64 but on a
variety of other versions too. apt-proxy is not involved. They are
present both with a Squid caching HTTP proxy and without. My
cron-driven updates usually see at least one instance of this error
They sometimes go away after a manual retry or three, making me wonder
if they are related to mirror update timing.