diff --git a/src/Engine/.Transport.php.swp b/src/Engine/.Transport.php.swp
new file mode 100644
index 0000000..a7e8943
Binary files /dev/null and b/src/Engine/.Transport.php.swp differ
diff --git a/src/Engine/Engine.php b/src/Engine/Engine.php
index 86bfb43..6054ca6 100644
--- a/src/Engine/Engine.php
+++ b/src/Engine/Engine.php
@@ -80,17 +80,11 @@ public function dealRequest($err, $success, $req, $res)
 
     protected function sendErrorMessage($req, $res, $code)
     {
-        $headers = array('Content-Type'=> 'application/json');
+        $headers = array(
+            'Content-Type'=> 'application/json',
+            'Access-Control-Allow-Origin' => '*'
+        );
 
-        if(isset($req->headers['origin']))
-        {
-            $headers['Access-Control-Allow-Credentials'] = 'true';
-            $headers['Access-Control-Allow-Origin'] = $req->headers['origin'];
-        } 
-        else 
-        {
-            $headers['Access-Control-Allow-Origin'] = '*';
-        }
         $res->writeHead(400, '', $headers);
         $res->end(json_encode(array(
             'code' => $code,
@@ -222,10 +216,27 @@ public function dealWebSocketConnect($err, $success, $req, $res)
             return;
         }
 
+
         if(isset($req->_query['sid']))
         {
+            if(!isset($this->clients[$req->_query['sid']]))
+            {
+                self::sendErrorMessage($req, $res, 'upgrade attempt for closed client');
+                return;
+            }
+            $client = $this->clients[$req->_query['sid']];
+            if($client->upgrading)
+            {
+                self::sendErrorMessage($req, $res, 'transport has already been trying to upgrade');
+                return;
+            }
+            if($client->upgraded)
+            {
+                self::sendErrorMessage($req, $res, 'transport had already been upgraded');
+                return;
+            }
             $transport = new WebSocket($req);
-            $this->clients[$req->_query['sid']]->maybeUpgrade($transport);
+            $client->maybeUpgrade($transport);
         }
         else
         {
diff --git a/src/Engine/Socket.php b/src/Engine/Socket.php
index 267ab64..4672038 100644
--- a/src/Engine/Socket.php
+++ b/src/Engine/Socket.php
@@ -6,6 +6,7 @@ class Socket extends Emitter
 {
     public $id = 0;
     public $server = null;
+    public $upgrading = false;
     public $upgraded = false;
     public $readyState = 'opening';
     public $writeBuffer = array();
@@ -29,6 +30,7 @@ public function __construct($id, $server, $transport, $req)
 
     public function maybeUpgrade($transport)
     {
+        $this->upgrading = true;
         $this->upgradeTimeoutTimer = Timer::add(
             $this->server->upgradeTimeout, 
             array($this, 'upgradeTimeoutCallback'),
@@ -53,11 +55,11 @@ public function onUpgradePacket($packet)
             $this->upgradeTransport->send(array(array('type'=> 'pong', 'data'=> 'probe')));
             //$this->transport->shouldClose = function(){};
             Timer::del($this->checkIntervalTimer);
-            $this->checkIntervalTimer = Timer::add(0.1, array($this, 'check'));
+            $this->checkIntervalTimer = Timer::add(0.5, array($this, 'check'));
         }
         else if('upgrade' === $packet['type'] && $this->readyState !== 'closed')
         {
-            $this->cleanup();
+            $this->upgradeCleanup();
             $this->upgraded = true;
             $this->clearTransport();
             $this->setTransport($this->upgradeTransport);
@@ -74,7 +76,7 @@ public function onUpgradePacket($packet)
         {
             if(!empty($this->upgradeTransport))
             {
-                $this->cleanup();
+                $this->upgradeCleanup();
                 $this->upgradeTransport->close();
                 $this->upgradeTransport = null;
             }
@@ -83,13 +85,17 @@ public function onUpgradePacket($packet)
     }
 
 
-    public function cleanup()
+    public function upgradeCleanup()
     {
+        $this->upgrading = false;
         Timer::del($this->checkIntervalTimer);
         Timer::del($this->upgradeTimeoutTimer);
-        $this->upgradeTransport->removeListener('packet', array($this, 'onUpgradePacket'));
-        $this->upgradeTransport->removeListener('close', array($this, 'onUpgradeTransportClose'));
-        $this->upgradeTransport->removeListener('error', array($this, 'onUpgradeTransportError'));
+        if(!empty($this->upgradeTransport))
+        {
+            $this->upgradeTransport->removeListener('packet', array($this, 'onUpgradePacket'));
+            $this->upgradeTransport->removeListener('close', array($this, 'onUpgradeTransportClose'));
+            $this->upgradeTransport->removeListener('error', array($this, 'onUpgradeTransportError'));
+        }
         $this->removeListener('close', array($this, 'onUpgradeTransportClose'));
     }
 
@@ -101,15 +107,18 @@ public function onUpgradeTransportClose()
     public function onUpgradeTransportError($err)
     {
         echo $err;
-        $this->cleanup();
-        $this->upgradeTransport->close();
-        $this->upgradeTransport = null;
+        $this->upgradeCleanup();
+        if($this->upgradeTransport)
+        {
+            $this->upgradeTransport->close();
+            $this->upgradeTransport = null;
+        }
     }
 
     public function upgradeTimeoutCallback($transport)
     {
-        echo("client did not complete upgrade - closing transport\n");
-        $this->cleanup();
+        //echo("client did not complete upgrade - closing transport\n");
+        $this->upgradeCleanup();
         if('open' === $transport->readyState)
         {
              $transport->close();
@@ -212,7 +221,8 @@ public function clearTransport()
     
     public function onClose($reason = '', $description = null)
     {
-        if ('closed' !== $this->readyState) {
+        if ('closed' !== $this->readyState) 
+        {
             Timer::del($this->pingTimeoutTimer);
             Timer::del($this->checkIntervalTimer);
             $this->checkIntervalTimer = null;
@@ -229,9 +239,11 @@ public function onClose($reason = '', $description = null)
             $this->request = null;
             $this->upgradeTransport = null;
             $this->removeAllListeners();
-            $this->transport->removeAllListeners();
-            $this->transport = null;
-           
+            if(empty($this->transport))
+            {
+                $this->transport->removeAllListeners();
+                $this->transport = null;
+            }
         }
     }
     
diff --git a/src/Engine/Transports/Polling.php b/src/Engine/Transports/Polling.php
index 09407a6..c0d4468 100644
--- a/src/Engine/Transports/Polling.php
+++ b/src/Engine/Transports/Polling.php
@@ -58,15 +58,18 @@ public function onPollRequest($req, $res)
 
     public function pollRequestOnClose()
     {
-        $this->pollRequestClean();
         $this->onError('poll connection closed prematurely');
+        $this->pollRequestClean();
     }
     
     public function pollRequestClean()
     {
-        $this->req->res = null;
-        $this->req->onClose = $this->req->cleanup = null;
-        $this->req = $this->res = null;
+        if(isset($this->req))
+        {
+            $this->req->res = null;
+            $this->req->onClose = $this->req->cleanup = null;
+            $this->req = $this->res = null;
+        }
     }
 
     public function onDataRequest($req, $res) 
@@ -79,8 +82,6 @@ public function onDataRequest($req, $res)
             return;
         }
 
-        $isBinary = 'application/octet-stream' == $req->headers['content-type'];
-
         $this->dataReq = $req;
         $this->dataRes = $res;
         $req->onClose = array($this, 'dataRequestOnClose');
@@ -118,20 +119,11 @@ public function dataRequestOnEnd ()
         $this->onData($this->chunks);
 
         $headers = array(
-            // text/html is required instead of text/plain to avoid an
-            // unwanted download dialog on certain user-agents (GH-43)
            'Content-Type'=> 'text/html',
-           'Content-Length'=> 2
+           'Content-Length'=> 2,
+           'X-XSS-Protection' => '0',
         );
 
-        // prevent XSS warnings on IE
-        // https://github.com/LearnBoost/socket.io/pull/1333
-        $ua = $this->dataReq->headers['user-agent'];
-        if ($ua && (strpos($ua, ';MSIE') || strpos($ua, 'Trident/'))) 
-        {
-            $headers['X-XSS-Protection'] = '0';
-        }
-
         $this->dataRes->writeHead(200, '', $this->headers($this->dataReq, $headers));
         $this->dataRes->end('ok');
         $this->dataRequestCleanup();
@@ -170,7 +162,8 @@ public function onClose()
     }
 
     public function send($packets) 
-    {
+    {   
+        $this->writable = false;
         if($this->shouldClose) 
         {
             echo('appending close packet to payload');
@@ -186,26 +179,25 @@ public function write($data)
     {
         $this->doWrite($data);
         call_user_func($this->req->cleanup);
-        $this->writable = false;
     }
 
     public function doClose($fn) 
     {
        if(!empty($this->dataReq)) 
        {
-           echo('aborting ongoing data request');
+           //echo('aborting ongoing data request');
            $this->dataReq->destroy();
        }
 
        if($this->writable)
        {
-           echo('transport writable - closing right away');
+           //echo('transport writable - closing right away');
            $this->send(array(array('type'=> 'close')));
            call_user_func($fn);
        }
        else
        {
-           echo('transport not writable - buffering orderly close');
+           //echo("transport not writable - buffering orderly close\n");
            $this->shouldClose = $fn;
        }
     }
diff --git a/src/Engine/Transports/PollingJsonp.php b/src/Engine/Transports/PollingJsonp.php
index a57c8e6..ec6c272 100644
--- a/src/Engine/Transports/PollingJsonp.php
+++ b/src/Engine/Transports/PollingJsonp.php
@@ -39,7 +39,7 @@ public function doWrite($data)
             'Content-Length'=> strlen($data),
             'X-XSS-Protection'=>'0'
         );
-
+        if(empty($this->res)){echo new \Exception('empty $this->res');return;}
         $this->res->writeHead(200, '',$this->headers($this->req, $headers));
         $this->res->end($data);
     }
diff --git a/src/Engine/Transports/PollingXHR.php b/src/Engine/Transports/PollingXHR.php
index 80df403..9fcb9a3 100644
--- a/src/Engine/Transports/PollingXHR.php
+++ b/src/Engine/Transports/PollingXHR.php
@@ -28,16 +28,10 @@ public function doWrite($data)
         $content_length = strlen($data);
         $headers = array( 
             'Content-Type'=> $content_type,
-            'Content-Length'=> $content_length
+            'Content-Length'=> $content_length,
+            'X-XSS-Protection' => '0',
         );
-
-        // prevent XSS warnings on IE
-        // https://github.com/LearnBoost/socket.io/pull/1333
-        $ua = $this->req->headers['user-agent'];
-        if ($ua && (strpos($ua, ';MSIE') || strpos($ua, 'Trident/'))) 
-        {
-            $headers['X-XSS-Protection'] = '0';
-        }
+        if(empty($this->res)){echo new \Exception('empty this->res');return;}
         $this->res->writeHead(200, '', $this->headers($this->req, $headers));
         $this->res->end($data);
     }