Coupons accepting GV redemption codes -- sort of....

I had a customer email us this evening because the store told her that it had accepted the discount code she entered, but never took the amount off her order.

Problem is, there was no coupon with the code she sent to me. However, that was the redemption code for the Gift Certificate that we had sent her as a promotional tool a while back.

She entered the GV Redemption code into the box for the Coupon Discount Code during the checkout process and when she clicked on Continue, it told her that the discount had been applied to her order.

I tried it myself, put a GV redemption code in the Discount Coupon code box and instead of telling that it was an invalid code, it told me that the discount had been applied

(and of course, nothing had been applied).

I am using 1.3.8a

Any ideas on how to stop something like this from happening again?