These days, surviving the financial world we live in seems to require a fair bit of due diligence! I learnt this the hard way when I discovered that Capital One reports your highest ever balance rather than your credit limit to credit bureaus, thereby potentially denting your credit score. (source)
Let me back up for a second and explain how credit scores work. One of the big factors that affect your FICO score is your debt utilization rate. What that means is that if you have a balance of $1,000 on a card with a $5,000 credit limit, your debt utilization is 20% (1,000/5,000 * 100). You want that rate to be as low as possible for each of your cards - any time that rate goes up over 50%, your credit score gets dinged! That's why it's better to have your balance split between 5 cards than on one card, all things being equal.
What Capitol One does is to report your highest ever balance rather than your credit limit. So if you currently have $50 on your card, and this is the first month you've ever charged on this card, a credit bureau will think I've maxed out my card, even if my credit limit is $100,000 (ok, it isn't, but you get the idea!) WHAM! So, I will have to pay off the Capital One card, which I was lured to get by a 0% introductory APR, defeating the purpose of getting the card in the first place. BOO to Capital One!
One more thing - if you do pay off on a card, DO NOT close the account; this will end up hurting your score, because your overall debt utilization just went up!