ASP.NET

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.

PageStatePersister _pers;
    protected override PageStatePersister PageStatePersister
    {
        get
        {
            if (_pers == null)
                _pers = new SessionPageStatePersister(this);
            return _pers;
        }
    }