I applied for Citi XXXX with XXXX month 0 % promotional APR and charged on the card on XX/XX/XXXX. On the statement with due date XXXX, it stated the promotion would end in XXXX billing periods. Originally I planned to pay off the full balance by XXXX. However, I forgot about it when I made payment on XX/XX/XXXX and there was remaining balance on the account, and only realized when I received the statement with over {$100.00} interest charge, calculated with the regular interest rate of 18.99 %. To prevent further interest charge, I paid the full balance by the due date. Out of my surprise, I received yet another interest charge close to {$100.00}. After calling the customer service, I was told if there was any balance, it required paying the full balance for XXXX months to prevent any further interest charge. 
Afterwards I read carefully on the statements, and realized that when the balance was not paid off on XX/XX/XXXX, Citi started to accrue the interest, not from XX/XX/XXXX, but from previous due date of XX/XX/XXXX. This calculation may be correct if the interest rate of previous month was already 18.99 %. But in this case, the interest rate of previous month should still be the promotional rate of 0.0 %. So Citi actually collected XXXX more month of interest. 
Citi may argue the promotional rate already ended even earlier. That is exactly the trick they play. Unlike other credit card companies that specifically states the date the promotional rate ends, Citi ambiguously states the rate ends in a certain billing period, with no specific date given. So Citi may argue the promotional rate ends at beginning of the billing period, rather than end of the billing period as we customers would assume. 
For people to prevent from making the same mistake, I recommend applying for cards that states clearly when the promotional period starts and ends, and avoid credit cards vaguely describe how interest charge is calculated, so that you will not be surprised. 
