What is the cause of this exception in ASP.NET?  Obviously it is a viewstate exception, but I can't reproduce the error on the page that is throwing the exception (a simple two TextBox form with a button and navigation links).
FWIW, I'm not running a web farm.
Exception
  Error Message: Unable to validate
  data.
  
  Error Source: System.Web
  
  Error Target Site: Byte[]
  GetDecodedData(Byte[], Byte[], Int32,
  Int32, Int32 ByRef)
Post Data
  VIEWSTATE:
  
  /wEPDwULLTE4NTUyODcyMTFkZF96FHxDUAHIY3NOAMRJYZ+CKsnB
  
  EVENTVALIDATION:
  
  /wEWBAK+8ZzHAgKOhZRcApDF79ECAoLch4YMeQ2ayv/Gi76znHooiRyBFrWtwyg=
Exception 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.default_aspx.ProcessRequest(HttpContext context)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
~ William Riley-Land