title: What does an "Unable to validate data" exception in System.Web.Configuration.MachineKeySection.GetDecodedData indicate
body: <p>I've developed a website testing on localhost Cassini and it has always run fine, now when I deploy to my webserver I intermittently get the following error:</p>
  
  <blockquote>
    <p>Global.Application_Error Error: Exception occurred during request:
    <a href="http://....blah.aspx" rel="noreferrer">http://....blah.aspx</a> Unable to validate data.    at
    System.Web.Configuration.MachineKeySection.GetDecodedData(Byte[] buf,
    Byte[] modifier, Int32 start, Int32 length, Int32&amp; dataLength)
           at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) HttpApplication.RaiseOnError => EventHandler.Invoke =>
    Global.Application_Error</p>
  </blockquote>
  
  <p>This is being caught in my global.asax.  I've searched around but can't find the cause.  This exception is being thrown in extremely simple and small webforms.  The site is not hosted within a webfarm.</p>
  
  <p>2 pages seem to cause this error intermittently and one of them stores a bool in the ViewState, but it is not modified apart from that.</p>
  
  <p>Has anyone come across this before?</p>