I had a similar experience. I wanted to cancel my old credit card but couldn't find a way to do it online. As there were no fees I just ignored it. Well, it turned out that my VPN still had it as the payment method and so I got a bill to please pay my negative balance (they require at least 10% to be paid off after a month).

Of course, I paid. But it turns out that I need to contact them to unlock the card after I had a negative balance. I kept postponing it and kept getting 0€ bills every month. After 3 months they told me they are sorry to see me go but they'd have to cancel my account because of a balance of -0€.

It was kinda funny but worked out well :D