5. Over-the-limit fees
The CARD Act mandated that issuers must allow you to opt in for potentially going over your credit limit -- and being charged a fee of up to $35 in the process. Banks says the easiest way to avoid this fee is to not sign up for it in the first place.
Giving the issuer permission to let you exceed your credit limit not only sets you up for a fee, but it also can have negative repercussions for your credit score. A full 30 percent of your FICO score is based on amounts owed, part of which comes from credit utilization, or how much of your available credit you've used. If you exceed your limit, you've effectively gone "upside-down" on that ratio, which hurts your score.