Invoked by the Javascript tracker when the data received from the browser is not well-formed. 

if ( empty( self::$data_js[ 'id' ] ) || empty( self::$data_js[ 'op' ] ) ) {
	do_action( 'slimstat_track_exit_102' );
	self::$stat[ 'id' ] = -102;
	self::_set_error_array( __( 'Invalid payload string. Try clearing your WordPress cache.', 'wp-slimstat' ) );
	self::slimstat_save_options();
	exit('-102.0');
}