Major Description
There is a protocol recommendation that when we attempt to revalidate a cache entry, but we receive a response that has a Date header that's actually older than that of our current entry, we SHOULD revalidate again unconditionally with either max-age=0 or no-cache (since some upstream cache would appear to be out-of-date).
http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.2.6