What happens when someone saves their cart... and then an item that they have saved in their cart is discontinued (so it's marked as "inactive" in store admin), then they come back later, load their cart, and check out?
When you deactivate a product the cart doesnt sweep the shopping cart table and remove products you have deactivated. To be safe you should write a job that periodically reads your deactivated products and deletes them from the shopping cart table.