A cache that receives an incomplete response (for example, with fewer bytes of data than specified in a Content-Length header) may store the response. However, the cache MUST treat this as a partial response. Partial responses may be combined as described in section 13.5.4; the result might be a full response or might still be partial. A cache MUST NOT return a partial response to a client without explicitly marking it as such, using the 206 (Partial Content) status code. A cache MUST NOT return a partial response using a status code of 200 (OK).
If a cache receives a 5xx response while attempting to revalidate an entry, it may either forward this response to the requesting client, or act as if the server failed to respond. In the latter case, it MAY return a previously received response unless the cached entry includes the "must-revalidate" Cache-Control directive (see section 14.9).