I made a really stupid mistake two months ago.
I don't automate my credit card payments, but I submit payments every month a few days after the statement closing date.
When my credit card bill was issued online in April, I went to pay it like normal. I selected the option to pay the whole thing, recorded the transfer, and it was done.
Then, when my May statement came, I noticed something weird. An interest charge. Now that was unexpected since I paid off the whole card (and have always done so). It was only a few dollars, so I called the company to find out what was up.
Turns out that I wasn't paying attention when I submitted the online payment. Yes, I selected the option to pay the whole card, but I had logged onto the system too soon after the statement was generated, and it was showing the total amount that I owed on my March statement, not my April statement. So while I paid more than enough to meet the minimum payment, I hadn't actually paid off the card as I thought. Which meant that I had interest due in May. And in June, in fact, as that interest just keeps accumulating, and had accumulated for the few days between the statement closing date in May and my subsequent payment.
This was my own fault for not paying close enough attention to the numbers. The payment didn't zero out my credit card when I paid it because I already had additional charges on the card (I use rewards card for EVERYTHING). So I just didn't notice.
Lesson learned. Thankfully, it was a relatively inexpensive lesson. Less than $20 all told. But frustrating. I will be sure to always make sure that I'm paying the proper balance from now on.