NOTE: these would be better checks on the extra stock check:

Code:
//        if ( (zen_get_products_stock($products[$i]['id']) - ($_SESSION['cart']->in_cart_mixed($products[$i]['id']) - $_SESSION['cart']->get_quantity($products[$i]['id']))) <= 0) {
          if ($_SESSION['cart']->in_cart_mixed($products[$i]['id']) > zen_get_products_stock($products[$i]['id'])) {
Code:
//      if ( (zen_get_products_stock($order->products[$i]['id']) - ($_SESSION['cart']->in_cart_mixed($order->products[$i]['id']) - $_SESSION['cart']->get_quantity($order->products[$i]['id']))) <= 0) {
      if ($_SESSION['cart']->in_cart_mixed($order->products[$i]['id']) > zen_get_products_stock($order->products[$i]['id'])) {