Someone sees something, so he says something. The person he says it to -- a policeman, a security guard, a flight attendant -- now faces a choice: ignore or escalate. Even though he may believe that it's a false alarm, it's not in his best interests to dismiss the threat. If he's wrong, it'll cost him his career. But if he escalates, he'll be praised for "doing his job" and the cost will be borne by others. So he escalates. And the person he escalates to also escalates, in a series of CYA decisions. And before we're done, innocent people have been arrested, airports have been evacuated, and hundreds of police hours have been wasted.

How LED signs become a national emergency. Probably one of the most accurate things Schneier has written on anti-terrorism security. There are some interesting links from this article, including a campaign urging people to report suspicions of child abuse that simply shows a man holding hands with a child along with the text "It doesn't feel right when I see them together" and a phone number.

Schneier on Security: The War on the Unexpected