title: Why am I only *randomly* getting "Validation of viewstate MAC failed" in a single server environment?
body: <p>Yes, there are a million questions and threads on this particular error, with many well-documented solutions, which I will definitely try.  But first I want to understand the problem for my particular situation/environment.</p>
  
  <p>In my situation, this error seems to be entirely random.  I will go weeks without seeing it, then one day I'll log into my site and get the error.  Refresh the page, login again, and no problem.  During these periods in between incidents, nothing particularly special has happened to the web server.</p>
  
  <p>And that's the other thing, the error says "If this application is hosted by a Web Farm or cluster...", but it's not.  It's just one single Windows Server 2008.</p>
  
  <p>So, I understand the general solution is to add a set of static machine keys to the web.config file --- but why does this happen seemingly at random, and why does it happen to my site which is on a single Windows Server 2008 machine?</p>
  
  <p>I'm using ASP.NET 4 and my site is a Web Application running on IIS.</p>
  
  <p><strong>Error Message:</strong></p>
  
  <pre><code>Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that &lt;machineKey&gt; configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.
  </code></pre>
  
  <p><strong>Stack Trace:</strong></p>
  
  <pre><code>[ViewStateException: Invalid viewstate. 
      Client IP: ##::##:##:##:##
      Port: ##
      Referer: http://myserver/login.aspx
      Path: /login.aspx
      User-Agent: Opera/9.80 (Windows NT 6.1; WOW64; U; en) Presto/2.10.289 Version/12.01
      ViewState: /wEPDw...6OCX]
  
  [HttpException (0x80004005): Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that &lt;machineKey&gt; configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.]
     System.Web.UI.ViewStateException.ThrowError(Exception inner, String persistedState, String errorPageMessage, Boolean macValidationError) +235
     System.Web.UI.ViewStateException.ThrowMacValidationError(Exception inner, String persistedState) +14
     System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +274
     System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Deserialize(String serializedState) +4
     System.Web.UI.Util.DeserializeWithAssert(IStateFormatter formatter, String serializedState) +37
     System.Web.UI.HiddenFieldPageStatePersister.Load() +241
     System.Web.UI.Page.LoadPageStateFromPersistenceMedium() +106
     System.Web.UI.Page.LoadAllState() +43
     System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +8431
     System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +253
     System.Web.UI.Page.ProcessRequest() +78
     System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +21
     System.Web.UI.Page.ProcessRequest(HttpContext context) +49
     ASP.login_aspx.ProcessRequest(HttpContext context) +4
     System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +100
     System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean&amp; completedSynchronously) +75
  </code></pre>