User login is done in the validation, not the submit of the login form

so, this operation will break any other validators of the login form is overloaded. The problem can be replied if you modify any of the user login forms (normal or block) using form_alter, and you can add any validator to the form or to the form elements, but even if the elements set form_set_error on any of the form fields, the user_login_validate LOADS the user (by the execution of the user_authenticate API), instead of just doing a check and let the submit hook do the login and loading the user operations.

The problem is, that no module can break the login validation withouth a hard break of the form flow, instead of a single #validation callback.

I've done a resume here:
http://drupal.org/node/175813#comment-273704

Thanks :D