I received a warning email about this during the week. On checking it out I discovered that it can, and does, happen so I'll describe it here so you can gaurd against it:
You receive a phone call from "VISA" or "MasterCard" NOTE: I had a call when I purchased my IC-E2820, it was my Card Supplier (ie the company who send the bill) who contacted me, not MasterCard (the logo on the card).
The scammer gives you some details (name, badge number etc) that makes them sound genuine. They then ask if you've recently purchased something with your card (giving you the card number and details of the purchase, interesting point to note; my card company only quote the last 4 digits of the card number in their communications) for several hundred pounds. When you answer in the negative they give some details about a company they've been watching and say that the amount will be credited to your card and a fraud investigation started, they usually give you a reference number for the investigation too, in case you need to contact them about it!
They then ask to verify that you still have the card in your possession and ask you for the last 3 digits on the back of the card. NEVER give these 3 numbers in this situation! They are used to make phone/internet purchases and should only be used when you are ordering something when I had the call about the '2820 I asked if I knew where the card was (to check I still had it) and a security question that I had filled in on the application for the card.
What happens after you give those 3 numbers is orders are placed for goods using your card, often through bogus companies where the scammer "is" the company or for items that are then sold on. The first you know about it is the charges on your statement.
Some card issuers will not refund these charges, stating that you divulged the information to allow your card to be used. Pursuing this through the courts often has success but is time consuming.
In short if you are not using your card to purchase something then do NOT give the last 3 numbers on the back of your card.