Lesson: Think twice before adding someone to your credit card as a user.

"Adding an authorized user to your account means that you take full responsibility for all account activity including payments and all charges the authorized user makes, whether you approved them or not," says Denise Winston, money expert and owner of California-based Money Start Here, a financial education company.  And beware -- your evil twin (or ex significant other) may even secretly sabotage your financial future, she adds.

If you're a parent, the same risk applies if you add your child to your card, points out Clarky Davis, "The Debt Diva" for CareOne Credit Counseling, a debt relief service company. "You're on the hook for that money. You are responsible for damage they do." Sign teens up for a prepaid card instead so they can't overdraw the account, she suggests.