Oops

Long term stoozer and recently got a 0% card as I had some big purchases to put on it rather than use savings. Made a balance transfer, a purchase then a big balance transfer up to 95% of credit limit. I was surprised to see an interest charge appearing in my account, I hadn't read the previous statement and didn't know it was coming. When I phoned to enquire I was told I had the full 2 years to make 0% balance transfers but a limited time to make purchases which I'd gone past by 10 days when I made my purchase. This made me liable for interest on the whole balance but they kindly offered to wipe it if I pay back the purchase amount before my next payment is due. I consider myself an experienced player but it's so easy to be caught out.