Every month, I tend to pay off my entire credit card. Two months ago, I misread one of my smaller card bills (the one I use just for gasoline purchases), and inadvertently underpaid. The following month, I got hit with interest. I swallowed the pill and paid my bill off in full. Then, the next month, I got hit with interest again! I called the credit card company, and they introduced me to the world of residual interest.
Apparently, even though I paid the card in full, because I had underpaid one month I lost my “grace period” and was being charged interest for items purchased betweeen the date the statement was issued and the date I paid it off. The customer support person did credit me for the second month’s interest (although she warned me that I’m likely to see interest for at least one more month) and suggested that do a Google search to learn more. She was apologetic and clearly appalled.After learning about this scam, I am too!
For a good explanation, read this article at http://www.creditcards.com/credit-card-news/credit-card-residual-interest-1268.php