The Yusuf Islam incident earlier this week, in which the former Cat Stevens was denied entry into the U.S. when federal officials determined he was on the government's "no-fly" antiterror list, started with a simple spelling error. According to aviation sources with access to the list, there is no Yusuf Islam on the no-fly registry, though there is a "Youssouf Islam." The incorrect name was added to the register this summer, but because Islam's name is spelled "Yusuf" on his British passport, he was allowed to board a plane in London bound for the U.S. The Transportation Safety Administration alleges that Islam has links to terrorist groups, which he has denied; British foreign minister Jack Straw said the TSA action "should never have been taken."

The incident points up some of the real problems facing security personnel as they try to enforce the "no-fly" list. One issue is spelling; many foreign names have several different transliterations into English. And the sheer size of the list is daunting; thousands of names have been added in the last couple months, says one government official, bringing the total up to more than 19,000 names to look out for. That makes it difficult for airlines and government agencies to check all passengers. Within the past six months, several people on the no fly list have been mistakenly allowed to fly.

Still, the TSA is learning. It recently acknowledged that a Federal Air Marshall, unable to fly for weeks when his name was mistakenly put on the "no-fly" list, was in fact not a threat, and removed his name from the list.