Managing Your Own Checkbook

You've spent the last hour in the Apple® store and now you're ready to make your purchase. After swiping your check card, the cashier tells you that it's been rejected due to unavailable funds. You're not sure why since you just balanced your checkbook and you're absolutely positive that there's $250.00 in the account. You go around the corner to an ATM and check the balance only to find that it's actually $122.56. How is that possible?

What are some reasons that could have caused you to not have enough money in your account?