title: How I can fix MAC error in asp.net application?
body: <p>I have an ASP.NET application all things are working fine but after some minutes when I click on a button it gives me this error :</p>
  
  <blockquote>
    <p><strong>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.  Description: An
    unhandled exception occurred during
    the execution of the current web
    request. Please review the stack trace
    for more information about the error
    and where it originated in the code.</strong></p>
    
    <p>Exception Details:
    System.Web.HttpException: 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.</p>
  </blockquote>
  
  <p>How I can resolve this ?</p>