If you have a debit (aka “ATM”) card associated with your checking account chances are that it sports a MasterCard or Visa logo. You may use it to pay for purchases as if it is a credit card when the merchant does not use a debit keypad for entering your PIN to authenticate the transaction. You may also use it for most other transactions that require a credit card online.

This is very convenient, especially if you only want to spend real money in your checking account rather than accumulate debt on a credit card. But there’s a very good reason NOT to use your debit card as a credit card.

Last week, on a Friday afternoon with a storm bearing down on Boston that would keep normally-open banks closed until Monday, an acquaintance opened up a web service that keeps track of her checking account balance – where she discovered that her balance was now $0 because someone in Europe had obtained her debit card number and drained it with charges!

Of course there are laws that protect you from being responsible for false charges, and this person had never set foot in Europe so was obviously the victim of a crime. BUT…

While calling MasterCard put a hold on charges to the account they could not do anything about her checking account balance – only her bank could put that money back. And this bank had no off-hours method for customers to do that – they required her to go to a branch in person on Monday.

So instead of being left with a credit card that was temporarily unusable, she was left with no money at all except for whatever cash was in her wallet on Friday afternoon!

I do not know exactly how her debit card information was stolen, but here are some ways to minimize your exposure – do not use a debit card for:

– Online purchases: if the website’s security is breached, or your network traffic is being “sniffed” by that guy at the corner table in your local coffeehouse, you’ve handed over the keys to your cash.

– Restaurants, hotels, or any other place where your card gets taken out of your sight to be processed: all that staff person needs to do is copy your account number and CVV number from the card to go home later and have an online shopping spree. Or even worse, sell your information to a reseller -which is probably how her card ended up being used in Europe.

– Any place that wants to keep your card information “on file” for later charges: making it available to be stolen by staff or used for items you have not authorized (“But I DIDN’T empty the mini-bar!”)

– Recurring payments: You decide to quit that gym but can’t get them to stop taking out the monthly fee.

– Anything that is going to take out money before you get the item, e.g., for later delivery, a trip in the future, etc.: If you don’t get the item, the trip gets canceled, etc. you are out real money unless you can get a refund.

If you are trying to avoid carrying a debt load, get yourself ONE credit card and always pay it off in full each month – you can specifically request a maximum credit line that is no more than what you can afford to pay off so that you won’t be tempted to splurge.