You arrive at your hotel and check in at the front desk. When checking in, you give the front desk your credit card information (for all the charges for your room).
You get to your room and settle in. Someone calls the front desk and asks, for example, for room 620 (which just happens to be your room). Your phone rings in your room. You answer, and the person on the other end says the following, 'This is the front desk. When checking in, we came cross a problem with your charge card information. Please re-read me your credit card number and verify the last 3 digits numbers on the reverse side of your charge card.'
Not thinking anything unusual, you might give this person your information, since the call seems to come from the front desk. But actually, it is a scam. Someone is calling from someplace other than the hotel front desk. They ask for a random room number, then, sounding very professional, ask you for credit card information and address information. They are so smooth, you will think you are talking to the front desk.
If you ever encounter this problem in your travels, tell the caller that you will come down to the front desk to clear up any problems. Then, go to the front desk and ask if there was a problem. If there was none, inform the manager of the hotel that someone acting like a front desk employee called to scam you of your credit card information.