Invalid postback
or callback argument. Event validation is enabled using <pages
enableEventValidation="true"/> in configuration or <%@ Page
EnableEventValidation="true" %> in a page. For security
purposes, this feature verifies that arguments to postback or callback events
originate from the server control that originally rendered them. If the
data is valid and expected, use the
ClientScriptManager.RegisterForEventValidation method in order to register the
postback or callback data for validation.
