When `http.Transport` sends an HTTP/1 CONNECT request containing a non-empty `Request.Body`, the body is written directly to the connection without framing following the request headers. If the server responds to the CONNECT request with a non-2xx keep-alive response, the connection is returned to the idle pool. Since CONNECT requests are not expected to have a request body, the server may treat the remaining body bytes as a subsequent pipelined HTTP/1.1 request. This results in a desynchronized pooled connection, causing the next user of that connection to receive the response intended for the injected request. In reverse proxies, such as `httputil.ReverseProxy`, that utilize a shared `Transport`, this behavior can lead to cross-user response poisoning.