Did you shop at Walmart on 31 March 2004?

If you did, then as this article: Wired News: The Bigger They Are ... indicates, you might want to check your credit/debit card statement. To quote,

DENVER -- A computer hardware problem caused more than 800,000 credit and debit card transactions to be double- or triple-billed last week at Wal-Mart stores nationwide, according to officials at First Data Corp., which handled the electronic payments.