(Smart Money) — The young clerk’s sympathetic smile hardly made me feel better. My debit card had just been declined at a local boutique, and she was clearly welcoming me to the world of financial dingbats who can’t keep track of their account balance. Problem was, I had more than enough in my checking account to cover the charge. But thanks to my “wild” spending spree (three small purchases in an hour is pretty wild for me), my bank’s automated fraud-alert system had triggered an account freeze. Used to be, cardholders got hit with a freeze only when they did something really unusual, like buying diamonds in Zimbabwe. But lately, banks seem a little quick on the trigger. While they guard their specific strategies like state secrets, consultants say that these days you might get the freeze if you use the wrong ATM or download an app. Card companies have good reason to crack down, says Julie McNelley, senior risk analyst for Aite Group. Fraud losses have risen about 10 percent since 2006, to an estimated $9 billion in 2010; struggling banks can hardly afford their $18 million CEO pay packages, let alone reimbursements for unauthorized purchases. The problem, of course, is that fraud detection ain’t easy. Even when the system is working well, just one in 15 suspected cases turns out to be an actual fraud. The rest of the time, it’s a false alarm—and an embarrassing annoyance when your card is declined at a client dinner.