No Error Messages For Failed Form Submissions

This behavior seems to be triggered when the function form_builder is not setting the process_input flag in form.inc:

if ($form_state['programmed'] || (!empty($form_state['input']) && (isset($form_state['input']['form_id']) && ($form_state['input']['form_id'] == $form_id)))) {
      $form_state['process_input'] = TRUE;
    }
    else {
      $form_state['process_input'] = FALSE;
    }

Hence, none of the validation code in drupal_process_form is executed. There is a comment in form.inc regarding this:

drupal_process_form($form_id, $form, $form_state);

// If this was a successful submission of a single-step form or the last step
// of a multi-step form, then drupal_process_form() issued a redirect to
// another page, or back to this page, but as a new request. Therefore, if
// we're here, it means that this is either a form being viewed initially
// before any user input, or there was a validation error requiring the form
// to be re-displayed, or we're in a multi-step workflow and need to display
// the form's next step. In any case, we have what we need in $form, and can
// return it for rendering.

return $form;