title: Django-paypal: IPN requests with Invalid Postback
body: <p>I am building an e-commerce site using <code>django-paypal</code> to accept Paypal payments. </p>
  
  <p>95% of the time, everything works flawlessly, users are able to pay, sandbox or not, and receive what they purchased.</p>
  
  <p>The other 5% of the time, however, instead of '<code>VERIFIED</code>' or '<code>INVALID</code>' as the response code, I get: <code>Invalid postback. (&lt;html&gt; &lt;body&gt; Fatal Failure &lt;br&gt; &lt;/body&gt; &lt;/html&gt; )</code></p>
  
  <p>This prevents our program from knowing what to do, and so the payment gets verified on our side EVEN THOUGH the payment is marked as 'complete' in the IPN and on Paypal's side.</p>