/**
 * Implements hook_commerce_checkout_pane_info_alter
 */
function checkout_forms_commerce_checkout_pane_info_alter(&$checkout_panes) {
    if(isset($checkout_panes['customer_profile_appointment_d'])){
        $checkout_panes['customer_profile_appointment_d']['callbacks']['checkout_form_validate'] = 'checkout_forms_commerce_checkout_form_validate';
    }
}

/**
 * Checkout pane validation override to validate customer profile and appointment
 */
function checkout_forms_commerce_checkout_form_validate($form, &$form_state, $checkout_pane, $order) {

    $first_name = $form_state['values']['customer_profile_appointment_d']['field_first_name_']['und'][0]['value'];
    $last_name = $form_state['values']['customer_profile_appointment_d']['field_last_name']['und'][0]['value'];

    //Some validation tests ... kept it simple for brevity.
    if (is_numeric($first_name)) {
        form_set_error(customer_profile_appointment_d . '][field_first_name_', t('Numeric characters are not allowed.'));
    }

    //Some validation tests ... kept it simple for brevity.
    if (is_numeric($last_name)) {
        form_set_error('customer_profile_appointment_d][field_last_name', t('Numeric characters are not allowed.'));
    }
    return TRUE;
}