The FAPI allows you to do this without altering the $_SESSION messages array by using #limit_validation_errors.

/**
 * Implements hook_form_alter().
 */
function my_module_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'my_form_id') {
    $form['my_textfield'] = array(
      '#type' => 'textfield',
      '#title' => t('Some title'),
      '#element_validate' => array('my_element_validate'),
    );
    $form['preview'] = array(
      '#type' => 'button',
      '#value' => t('Preview'),
      '#limit_validation_errors' => array(array('my_textfield')),
    );
  }
}

/**
 * My textfield element validate handler.
 */
function my_element_validate($element, &$form_state, $form) {
  // Check the value here.
  // Checking if the element has a value can be set in the field, but just to demo...
  if (empty($element['#value'])) {
    form_error($element, t('This field is required.'));
  }
}

The FAPI allows you to do this without altering the $_SESSION messages array by using #limit_validation_errors.

/**
 * Implements hook_form_alter().
 */
function my_module_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'my_form_id') {
    $form['my_textfield'] = array(
      '#type' => 'textfield',
      '#title' => t('Some title'),
      '#required' => TRUE,
      '#element_validate' => array('my_element_validate'),
    );
    $form['preview'] = array(
      '#type' => 'button',
      '#value' => t('Preview'),
      '#limit_validation_errors' => array(array('my_textfield')),
    );
  }
}

/**
 * My textfield element validate handler.
 */
function my_element_validate($element, &$form_state, $form) {
  // Check the value here.
  if ($element['#value'] == '123') {
    form_error($element, t('123 not allowed!'));
  }
}