Right now any coupon code that get's attached to an order will be considered used.
For most cases, this is a pretty poor behavior.
Consider an anonymous user that gets a coupon code through a purchase and wants to use the code. He finds stuff to buy, enters the code but is note sure if he wants to get something else and doesn't complete the order.
He then opens the site the next day
• After having cleared his cookies
• On a different browser/computer
His code (1 time use only) is now considered used, and there's a good chance he even can't access the order with the code on it - it's gone forever. I think we can do better than this.