title: Reasons why viewstate validation fails (but only sometimes)
body: <p>The following message appears in our log:</p>
  
  <blockquote>
    <p>The control tree into which viewstate
    is being loaded must match the control
    tree that was used to save viewstate
    during the previous request. For
    example, when adding controls
    dynamically, the controls added during
    a post-back must match the type and
    position of the controls added during
    the initial request.</p>
  </blockquote>
  
  <p>Well, that's pretty clear, and alot has been written about this subject. However, I cannot reproduce this behavior, and I only find a couple thousand errors per day in the logs; so it's probably isn't that obvious.</p>
  
  <p>The page in question has an <code>UpdatePanel</code> and loads one of three usercontrols depending on the querystring.</p>
  
  <p>Is there any known issue with some clients, or any other possible explanation?</p>