User defined error message do not work

[eluser]rm_beginner[/eluser]

Code:

$this->form_validation->set_rules('callback_show_error_message', 'username', 'Username');

try to put your callback right side.

Code:

$this->form_validation->set_rules( 'username', 'Username', 'callback_show_error_message');

hope it works.