That means you didn't thoroughly merge your customized english.php with the master english.php, and you're missing at least this line:
Code:
define('WARNING_SHOPPING_CART_COMBINED', 'NOTICE: For your convenience, your current shopping cart has been combined with your shopping cart from your last visit. Please review your shopping cart before checking out.');
And, if you're missing that one, you should probably triple-check *all* your language files, and be sure that you've merged *all* your customizations with *all* the updates, in *all* your language files, template files, override modules, etc.