It doesn’t happen often in my experience, but there are times when a person has a negative balance on their credit card. This might happen because you bought something and got a refund after the statement and payment dates so the returned money goes onto your next statement.
This happened to me recently when I bought a ticket in May to visit Florida in September. My wife and I decided not to go and got a full refund due to some flexible rules of our ticket. I had a negative balance on the card so I called them (using my Sapphire Preferred phone number so I was routed right to a person) and easily requested a refund. Not five days later (including a weekend in between) I found a letter in the mail explaining why I was receiving the check. It’s nice to know credit card companies can really work for you when you want them to.