Coupon Code sometimes results in a negative credit balance

[efficiondave]

We've had a Coupon Code in place for several years now and generally it works great. But sometimes, rather than decreasing the amount the user pays, we end up with a negative Credit Balance / Remaining Balance. Prior to this year, it only happened 4 times. But it's happening a lot more frequently since Feb. 1, 2013. Since then, out of 201 orders that used the coupon code, 38 have a Total Payments amount that is higher than the Total Charges resulting in a credit balance. So, it's only happening around 20% of the time.

I've tried to find a pattern in the orders with the problem vs. the order handled correctly and haven't had any luck yet.

Here's what the typical History and Notes page looks like for these orders:
2/4/2013 3:44:05 PM
system The order has been paid, a credit balance of -$5.00 remains.
2/4/2013 3:44:05 PM
system A payment capture for $57.00 has been received. No further captures possible.
2/4/2013 3:44:03 PM
system A payment authorization for $57.00 is being requested.
2/4/2013 3:43:48 PM
system The order has been submitted.

Has anyone else seen this? Any idea what might be causing it?

[efficiondave]

This is still happening intermittently and I still haven't been able to find any pattern for when or why it occurs. Any ideas?

[trozzi]

Are there multiple lines in the order when the credit balance exists?

[efficiondave]

Not sure what you mean by "multiple lines in the order". Could you clarify?

[trozzi]

Are there multiple products in the basket when the coupon code is applied when you get the credit ?

[efficiondave]

It happens for both single item orders and multiple item orders. I suppose I don't really know if the user added or removed items after setting the code. And that brings up the valid idea that perhaps the difference between it working correctly or not is, at what point the user enters the code. I'm going to test with those things in mind... Thanks!

[efficiondave]

Found it! We've made substantial changes to the checkout workflow so I doubt this will affect others but here's the summary anyway...
We added a review page after the Payment page. Users can enter a PROMO code on either page. On the Review page, entering the Promo code would update the order total but would not affect the amount to be billed against the Credit Card.

Thanks to trozzi for the nudge in the right direction.