I've had a debit card number I never even used online stolen, presumably by some unscrupulous employee in a brick-and-mortar retailer. It's not fun, obviously, but disputing a charge (or charges) is stupidly easy -- and that's if the bank doesn't catch the activity before you even see it, which they usually do.
I just always use a credit card for purchases now, rather than a debit card, so there's no chance of me temporarily being out any money. I used to use temporary account numbers but it's such a non-issue with credit cards I don't even bother anymore.