Thanks Amos, The thing is that the server replied with 304 and not 5xx. "Revalidation failed" was added to the replay header even if not reciving the 5xx response.
Looking at the code 3.2.0.10 it seems to be the same for the warning: // add Warnings required by RFC 2616 if serving a stale hit if (http->request->flags.stale_if_hit && logTypeIsATcpHit(http->logType)) { hdr->putWarning(110, "Response is stale"); if (http->request->flags.need_validation) hdr->putWarning(111, "Revalidation failed"); } http->request->flags.need_validation is set to 1 in clientReplyContext::cacheHit() Just to be sure: we are not supposing to see the [Warning: 111 squid/3.2.0.5 "Revalidation failed"] if we got 304 replay from the server correct (regradless to the etags)?