I just paid off a card that had 0 % on balance transfer and purchases. I used it for purchases (was a cash rewards card, $100 bonus for first use)and just paid the monthly minimum til the 12 months was over. I then paid the full amount due, on time (actually early) prior to the end of the promotion period. So there was a 0 balance and had paid no interest for the 12 months. I continued to use the card estimating I had put about $250 on it in the next month. Got the bill. New balance was 320. Ok, I estimated wrong. About to pay it in full. Looked closer, NO!, they charged me $50 interest.
Called Chase and they did credit it back to me, but had I not looked closely I would have just paid it. I figure their 2 cycle finance charge calculations caused this since I had kept a balance month to month for the 0 % interest period. So if I do this again on another card with similar offer guess I should not use the card for the month after the promotion period or remember to check and call and complain.

I am currently doing this backwards – I have the money to pay off the 0% that I have saved up over time (it was a balance transferred from a real card) but I am keeping it in the bank until it is time to pay it off.