HSBC, Best Buy Credit Card Intrest Free for 12 Months, More Like Intrest Free for 2 Months! Nationwide

You took the time to write this post, it seems that if you took the time to call them and find out why you were charged interest we probably wouldn't be reading this right now.

There are actually two different best buy cards that can be applied for. One which does get 0% offers, and one which does not. Best Buy itself has a flier at their customer service desk to show this, however it seems their employees don't take their customers to this desk to fill out the applications. More than likely you applied for the Rewardzone mastercard, which does not get a 0% offer. The reason why the balance also goes up, rather than down, is that you may have checked the payment protection plan, known as Account Shield. This is actually a problem easily fixed, by going back to Best Buy, and applying for the store card, which is only good to use at Best Buy, unlike the Rewardzone card, which can be used elsewhere. Once approved for a store card, they can credit the Rewardzone card, and put the purchase on the store card. Unfortunately, it seems like a lot of Best Buy employees, also don't realize the differences in the cards, even though they also can read the differences at a desk in their own store.

Sounds like you either missed a payment or failed to make a minimum payment. That is the only way you are going to be charged the interest you originally weren't going to have to pay. Or if you didn't pay off your purchase by the end of the promotion, but seeing as you are at your 11th month, it's either of the first two.