title: Strange unhandled exception from asp.net application - Validation of viewstate MAC failed
body: <p>I don't know if anyone has seen this issue before but I'm just stumped. Here's the unhandled exception message that my error page is capturing. </p>
  
  <blockquote>
    <p>Error Message:  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>
    
    <p>Stack Trace:  at
    System.Web.UI.ViewStateException.ThrowError(Exception
    inner, String persistedState, String
    errorPageMessage, Boolean
    macValidationError)  at
    System.Web.UI.ObjectStateFormatter.Deserialize(String
    inputString)  at
    System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Deserialize(String
    serializedState)  at
    System.Web.UI.Util.DeserializeWithAssert(IStateFormatter
    formatter, String serializedState) at
    System.Web.UI.HiddenFieldPageStatePersister.Load()
    at
    System.Web.UI.Page.LoadPageStateFromPersistenceMedium()
    at System.Web.UI.Page.LoadAllState()
    at
    System.Web.UI.Page.ProcessRequestMain(Boolean
    includeStagesBeforeAsyncPoint, Boolean
    includeStagesAfterAsyncPoint)  at
    System.Web.UI.Page.ProcessRequest(Boolean
    includeStagesBeforeAsyncPoint, Boolean
    includeStagesAfterAsyncPoint) at
    System.Web.UI.Page.ProcessRequest() 
    at
    System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext
    context)  at
    System.Web.UI.Page.ProcessRequest(HttpContext
    context)  at
    ASP.generic_aspx.ProcessRequest(HttpContext
    context)  at
    System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
    at
    System.Web.HttpApplication.ExecuteStep(IExecutionStep
    step, Boolean&amp; completedSynchronously)</p>
    
    <p>Source: System.Web</p>
  </blockquote>
  
  <p>Anybody have any ideas on how I could resolve this? Thanks.</p>