This happened to me two weeks ago. Luckily we caught it (My bank did) really quickly....with only $300 in charges made.

They had, not only my debit card number....BUT...they had the CVV code off the back of it. Someone either had to get this info.from an online purchase I made....OR....a waiter/waitress (someone who's held my actual card) had to steal the information.

No other explanation I can think of. I'm just glad we caught it quickly. It could have gotten REAL ugly.