Comment 14 for bug 14486

Thanks for this additional info. It looks like your cable company is using a
transparent proxy (Cisco Application and Content Networking System Software
5.1.13). It seems to not check if the data on the server is more current that
the data in it's cache and that's why you see the error. There is no real good
workaround for that beside turning off If-Modified-Since and Last-Modified in
the apt source code. This is a pretty drastic measure that will lead to a lot of
additonal traffic on the servers and mirrors. Another option is to add a apt
variable that can turn Last-Modified off for people behind bad proxies.