When your credit card company stops a thief from charging fraudulent 
expenses to your card, you're thrilled. But what happens when they 
mistake you for the thief? 
7 reasons your credit card 
gets blocked With $6.89 billion in fraud losses in 2009, credit card 
companies eager to stem the tide are continually beefing up their 
anti-fraud measures, relying on sophisticated computer software to flag 
suspicious transactions. 
Trouble is, what looks like a red flag to a 
computer may just be you trying to make a mundane purchase. Then, all of
 a sudden, your card's declined, leaving you red-faced and frustrated. 
So
 what looks bad to your card company? Anything out of the ordinary. "The
 credit card companies -- Visa, MasterCard, American Express, Discover 
-- all have their own proprietary technologies that look for anomalies 
in your spending habits," says Robert Siciliano, a McAfee consultant and
 identity theft expert based in Boston. 
Siciliano suggests that each 
transaction is automatically analyzed for up to 200 different data 
points, everything from where you live to what you normally buy to how 
much you're spending, to determine the likelihood that you're the one 
actually making a particular charge. If the analysis doesn't add up, 
your card will be blocked and your next purchase declined.
What triggers a block 
1. Card
 issuers won't go on the record about specific red flags -- as Siciliano
 points out, "That'll only give the bad guys an edge." But according to 
experts and hapless cardholders who have experienced a block, these 
shopping habits may lead to hassles:
2. Shopping where 
you've never shopped before. "I've had calls from my card company 
saying, We've detected unusual activity.' It wasn't unusual, but it was
 a different pharmacy than the one I normally went to," says Denise 
Richardson, a certified identity theft risk management specialist and 
author of "Give Me Back My Credit!" 
3. Making several purchases
 quickly. Janis Badarau, of Lavonia, Ga., sometimes hits three grocery 
stores in a row to find what she needs and take advantage of sales. But a
 few months ago, she was so speedy that by the time she swiped her card 
at the third store, it was declined. "I called the bank when I got home,
 and they told me that shopping at three supermarkets within an hour or 
so was considered 'unusual activity,'" Badarau says. 
4. Charging something small, then something big. Criminals sometimes test 
the waters with a stolen card by charging a tiny amount -- say, a song 
on iTunes -- before moving on to a triple-digit purchase. That small-big
 pattern in your own buying habits may result in a declined card. 
5. Shopping away from your home base. That's especially common when 
you're moving. "If my billing address is Massachusetts and I'm buying a 
washer and dryer in Idaho, that's an anomaly, because why would I buy a 
washer and dryer in Idaho if I live in Massachusetts?" says Siciliano. 
6. Charging travel expenses. On the road, any purchase from gas to 
restaurant meals can trigger a block. While that's long been true for 
travelers abroad, it now happens domestically, too. "Once my travel to 
L.A. flagged it and I spent 20 minutes verifying transactions," says 
Traci Coulter, of New York City. When she asked what caused the card to 
be declined, she was told, "a taxi, a charge at the airport, in-air 
Wi-Fi and a rental car hold" -- all standard travel expenses. 
7. Buying things in different geographic regions on the same day. During a
 cruise, Janet Gillis, of Tampa, Fla., used a card to get money from an 
ATM on the ship, then she later made a purchase on-shore in Belize. For 
the rest of the trip, her card was declined. "Apparently, the ATM on 
board the ship is registered to a Miami location, and several hours 
later, I was purchasing something in Belize. To them, it looked 
suspicious because the transactions happened so close together," says 
Gillis. Online purchases to merchants in different parts of the world 
can trigger the same flag. 
    Dealing with billing issues. When
 Siciliano wanted to make an addition to an online purchase, he 
contacted the company, but the second transaction they tried to process 
was declined. The card issuer "thought that the merchant was taking 
advantage of my card number."