- Jad Chamcham pointed out a bug with connection re-use. If a connection had

CURLOPT_HTTPPROXYTUNNEL enabled over a proxy, a subsequent request using the
  same proxy with the tunnel option disabled would still wrongly re-use that
  previous connection and the outcome would only be badness.

lib/url.c

@@ -2758,6 +2758,7 @@ ConnectionExists(struct SessionHandle *data,
               is the checked one using the same host, port and type? */
       if(check->bits.proxy &&
          (needle->proxytype == check->proxytype) &&
+         (needle->bits.tunnel_proxy == check->bits.tunnel_proxy) &&
          Curl_raw_equal(needle->proxy.name, check->proxy.name) &&
          needle->port == check->port) {
         /* This is the same proxy connection, use it! */