The client had just had a customer service issue where one of their customers discovered many months later that her card was still being charged for her subscription even though the card had expired. She didn't know that many cards continue to work after they expire, and had never bothered to cancel her account.
So the client wanted me to make sure that we don't charge expired cards in the future. As far as I knew, this was the first time the issue had come up in the more than a year since we switched payment gateways, so I generated some new reports, and it turns out they had charged almost $4,000 to expired cards just in the previous month. Some of those customers would correct their card data if we notified them, but the reality of subscription businesses is that there's a lot of inertia involved. It's safe to say that the client would simply lose a fair portion of those dollars every month if we stopped charging expired cards with active subscriptions. Much better to refund the one customer/year who complains that they thought an expired card meant they unsubscribed than to forgo all that revenue. Of course they agreed and we didn't make the change.
FTR, this is the same client that a few months ago was complaining that I was the only programmer they'd ever worked with who bills them for email and phone conversations. I'll be reminding them of this case if it ever comes up again.