False positives are failures, too
bruce_schneier is dead on today:
The TSA likes to measure its success by looking at the forbidden items they have prevented from being carried onto aircraft, but that’s wrong. Every time the TSA takes a pocketknife from an innocent person, that’s a security failure. It’s a false alarm. The system has prevented access where no prevention was required.
Right. And some of you know how mad I’ve been about this issue for some time. Let’s crank up the wayback machine – three items from 2002 here….
lullaby pit 
A lady I knew was arrested and handcuffed for being in posession of a knife, fork, and spoon, but was allowed to board when she surrendered these weapons. When she got off the plane and went looking for a quarter to make a phone call she found her boxcutters, which had been overlooked.