Attacked by credit card vampires

This weekend I was paying my credit card bill and noticed that through an oversight I had underpaid my last month's bill by $60. What I couldn't figure out was how that one-month, $60 underpayment resulted in a finance charge of $20. I called up the credit card company and they were happy to explain it to me: as soon as you pay less than the full amount of a bill, you begin paying interest not just on the carryover amount, but also on anything you add to the card in the meantime. My card was one that gives you points for using it, so of course I put everything I could on the card, and they were charging me interest on each purchase from the second that I purchased it. In other words, vampires.

Maybe I'm the only person who didn't realize this is how the financing works, but it got me pretty mad. My vampire card is MBNA, they do a lot of deals with organizations so that the organizations can have their "own" cards. Watch out for them, and carry garlic.