If you used a Visa or MasterCard credit or debit card on Wednesday -- you may have been charged twice, or even three times. Officials say a computer hardware problem caused more than 800-thousand credit card and debit transactions to be double-billed or triple-billed. The transactions took place at stores around the country -- including Wal-Mart. A spokeswoman for the electronic payments processor First Data Corporation says the excess charges have been reversed. The reversals may not show up in some accounts until Tuesday. She says anyone who made a charge with either of the two cards on Wednesday "should check their statements." First Data is making phone calls to all of the affected customers. And it's publicizing a toll-free hotline of its own -- 888-893-0626.