Since funds are immediately taken out of your account when you use a debit card, any error on the bank’s or merchant’s part could ding you considerably. You could overdraw, incurring a fee that averages almost $30, and not be able to use your card for additional purchases. So take action. As soon as you notice a discrepancy, contact the financial institution with your receipt in hand, since it should accurately reflect how much money should have been debited from your account, says Greg McBride, a senior financial analyst for Bankrate.com. Already tossed the receipt? No matter. The bank will probably still initiate an investigation.