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