I hope this is not a repeat of any posts as I could not find any.
Running nopcommerce 1.8 on MS SQL 2008, arvixe hosted
If you add a few items to your shopping cart, then from the adminstration edit one of the products in the shopping cart and uncheck the "published" check box, then go back to your cart, it will list all the items and the one you "un-published" will have red text saying "not a published item". If you try to remove any items from your cart, nothing will happen until the non-published item is set to published again.
This happened when a customer kept a saved shopping cart for a few weeks then tried to come back to make purchase after the administrator had changed one of the items in the customers cart to not be published.