One suggestion for the feature versions

in file:

includes/modules/pages/checkout_login/header_php.php


comment this line:
$_SESSION['cart']->restore_contents();

so it looks:

//$_SESSION['cart']->restore_contents();

then add this:

Code:
if (SHOW_SHOPPING_CART_COMBINED > 0) {
          $zc_check_basket_before = $_SESSION['cart']->count_contents();
        }

        // bof: not require part of contents merge notice
        // restore cart contents
        $_SESSION['cart']->restore_contents();
        // eof: not require part of contents merge notice

        // check current cart contents count if required
        if (SHOW_SHOPPING_CART_COMBINED > 0 && $zc_check_basket_before > 0) {
          $zc_check_basket_after = $_SESSION['cart']->count_contents();
          if (($zc_check_basket_before != $zc_check_basket_after) && $_SESSION['cart']->count_contents() > 0 && SHOW_SHOPPING_CART_COMBINED > 0) {
            if (SHOW_SHOPPING_CART_COMBINED == 2) {
              // warning only do not send to cart
              $messageStack->add_session('header', WARNING_SHOPPING_CART_COMBINED, 'caution');
            }
 // check current cart contents count if required
            if (SHOW_SHOPPING_CART_COMBINED == 1) {
              // show warning and send to shopping cart for review
              $messageStack->add_session('shopping_cart', WARNING_SHOPPING_CART_COMBINED, 'caution');
              zen_redirect(zen_href_link(FILENAME_SHOPPING_CART, '', 'NONSSL'));
}
}
}
This way, if cart was combined (products added to cart before, but not purchased), the login page redirects to cart, and displays warning.