Problem on the comment form #validation and #submit I've tried to create a validation and submit functions on a custom comment form, using drupal 7.9,  but it's not working, and I don't know why.

In the following code, we can see the "dpm's" in the first function, but not the "dpm's" in the last ones.

function job_form_comment_node_job_form_alter(&amp;$form, &amp;$form_state) {
    $form['#validate'][] = 'job_comment_validate';
    $form['#submit'][] = 'job_comment_submit';
    dpm($form);
    dpm($form_state);
}


function job_comment_validate($form, &amp;$form_state) {
    dpm($form);
    dpm($form_state);
}

function job_comment_submit($form, &amp;$form_state) {
    dpm($form);
    dpm($form_state);
}


if I add a #validate or #submit it also doesn't call others drupal hook comments like presave, insert etc.

If I comment these lines, the hooks are called, but I can't use the validation or the submission functions in this case.

Do you have any idea on why this happens?
You're missing $form_id, the third parameter of the function. 

See hook_form_alter() reference on drupal.org. The problem you're experiencing might be a (weird) way of PHP reacting to that. 

In situations like this you may want to check for errors on the logs (Drupal, PHP, MySQL). Some times there are useful hints there to figure what's wrong.