I create an control extends the gridview, which implements ICallbackEventHandler interface. I use a client side timer event to repeatedly raise the call back event every 30 seconds. In the raiseCallBackEvent method, I raise another event which is handled on page. Then I use the javascript function that receives the call back result to update the gridview. Some time everything works well, but sometimes I got this error message: page viewstate is invalid? Could somebody tell me what cause this problem?

Thanks