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