Right now a coupon code is considered used when the user enters it in the Coupon Code section of the shopping cart and applies it to their order. If the user does not complete the transaction, the coupon is still considered used.
That means you could have coupon uses counting against the max use you set when the transactions have not been completed. Theoretically, you could have a coupon meet its maximum use yet not be used for any completed transaction at all! We've run into trouble when sending coupons to users, coupons that are only supposed to be used a couple times, and when they try to use them, they've already maxed out.
I propose that this be changed or at least provide an option in the coupon settings, whether to consider a coupon used when it's applied or when it's applied in a completed transaction.