Validation problem - Config | Callback | Controller

[eluser]Multisnet[/eluser]
My idea is to extend default CI form validation class, so i follow the user guide instructions:

class MO_Form_Validation extends CI_Form_Validation {

function MO_Form_Validation()
{
parent::CI_Form_Validation();
echo "LOAD - >MO_Form_Validation";
}
}

The extended class is loaded, however, my forms doesn't display errors anymore, It seems that the new class doesn't really extend the base CI class. I'm doing anything wrong?