very handle way to skip drupal form validation
$form['actions']['previous'] = array(
    '#type' => 'submit',
    '#value' => t('Previous'),
    '#limit_validation_errors' => array(),       // No validation.
    '#submit' => array('some_submit_function'),  // #submit required.
  );

limiting validation to only few field
 $form['actions']['previous'] = array(
    '#type' => 'submit',
    '#value' => t('Previous'),
    '#limit_validation_errors' => array(
      array('step1'),       // Validate $form_state['values']['step1'].
      array('foo', 'bar'),  // Validate $form_state['values']['foo']['bar'].
    ),
    '#submit' => array('some_submit_function'), // #submit required.
  );