Why no message when a promotion is already consumed?

Our users have pointed out a problem for us to research.

In our iStore, given this situation:

1.  A customer is given a promo code to use on their next order.

2.  The promo code has been set up to be used exactly once.

3.  The customer puts a few items in the shopping cart, and then saves the cart for another time.

4.  The next time they log in to place an order, they think they might rather use their promotion code on a different order, so they put some new items into the cart, and apply the promo code to it.  After all, they haven't "used" their promo code yet.