title: Setting ViewStateUserKey gives me a "Validation of viewstate MAC failed" error
body: <p>I have the following in my <code>BasePage</code> class which all my ASPX pages derive from:</p>
  
  <pre><code>protected override void OnInit(EventArgs e)
  {
      base.OnInit(e);
      ViewStateUserKey = Session.SessionID;
  }
  </code></pre>
  
  <p>I also have a <code>machineKey</code> set in <code>Web.config</code>. I don't think this error is because of a web farm because this happens on my dev machine too.</p>
  
  <p>My host has now upgraded to .NET 3.5 SP1. After this update, everytime I compile with the <code>ViewStateUserKey</code> setting above, I constantly get the "Validation of viewstate MAC failed" error on every postback.</p>
  
  <p>What am I doing wrong here? Is this setting even necessary anymore with the latest framework update?</p>