Setting a cookie translates to writing a header down the socket but it isn't written when SetCookie is called. It is written when you use the ResponseWriter as

Setting a cookie translates to writing a header down the socket but it isn't written when SetCookie is called. It is written when you use the ResponseWriter as a parameter for a writer method (io.WriteString, for instance). As SetCookie just adds a new header to an aliased map (Headers) and setting to a map shouldn't fail (how it could?), it makes sense to report any error when the HTTP response is written serialized to the socket, not before. At least, for me, as you can manipulate the response as you wish and you only need to worry about errors on write.