Link: http://trac.tools.ietf.org/wg/httpbis/trac/ticket/117
Origin: http://www.w3.org/mid/1208420896.30744.9.camel@hlaptop.henriknordstrom.net
Component: p2-semantics
This text currently is only specified for HEAD and not GET:
If the new field values indicate that the cached entity differs from the current entity (as would be indicated by a change in Content-Length, Content-MD5, ETag or Last-Modified), then the cache MUST treat the cache entry as stale.
It's an equally good rule for GET as for HEAD, and having them aligned would help getting rid of cornercases such as the #23 question.
A 200 response to GET is already going to replace the cache entry, since the reason the client is receiving a 200 response is because the cached one is considered invalid.