SACRAMENTO, Calif. -- The average American family carries credit card balances of more than $8,000, but KCRA-TV in Sacramento found that there are a lot of things credit card users still don't know about that powerful piece of plastic.

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Among those things is that a credit card company can change the terms of a contract with just 15 days notice. Creditors can change the due date or interest rate -- or anything else -- with just a written notice 15 days before the change goes into effect.

Also, a late payment on one bill can cause interest rates on other loans to go up. It's called universal default, and it means paying a gas bill late could cause your credit card rate to jump to the maximum.

And, shopping around for a deal could actually cost you.

"Say you shop around for the best interest rate, or you shop around for the best insurance premium, or you shop around for the best cell phone deal -- what that means is you get a lot of inquiries on your credit report," consumer advocate Birny Birnbaum said.

More inquiries can lead to a lower credit score, which can cost you more money in interest.

Experts said the best protection against credit hazards is to read credit cards notices and contracts thoroughly and pay bills on time.