title: What does existingResponse="PassThrough" mean in IIS?
body: <p>The documentation says</p>
  
  <blockquote>
    <p><code>existingResponse="PassThrough"</code></p>
    
    <p>Leaves the response untouched if an existing response exists.
    <a href="http://www.iis.net/configreference/system.webserver/httperrors#005">http://www.iis.net/configreference/system.webserver/httperrors#005</a></p>
  </blockquote>
  
  <p>But what does that mean by "existing response exists"?</p>
  
  <p>E.g. I want my <code>customErrors</code> handler to suppress the ASP.NET response, so that IIS would think that response doesn't exist. How would I do that?</p>