ESP8266 Community Forum

I don't have the precise RFC spec here, but I don't think it's valid to output response data while the POST send is still going on. (If I'm wrong there, I ofcourse would love to hear that.) Most browsers would accept it just fine, but I feel uncomfortable building in a feature that makes it easy to go against the spec. So no, sorry, for now you'll have to set an error flag in the CGI and handle it that way.

Mhh, I hadn't considered this and in most cases it's pretty irrelevant, specially for error responses that fully fit in pretty much any TCP buffer. I also don't see any real attention paid to this in rfc1945, at least that I can find. At best it's implied by the mention that HTTP is a request-response protocol. But it's your call