title: Viewstate verification failed. Reason: The viewstate supplied failed integrity check
body: <p>It's been a while that I'm getting "Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that  configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster." error on GridView post-backs.</p>
  
  <p>I've defined the same machine-keys on my sites but I <strong>(sometimes)</strong> get the message.</p>
  
  <p>Any clue?</p>