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President Obama declared on December 29th that a “systematic failure” had allowed 23-year-old Umar Farouk Abdulmutallab to board a US-bound aircraft with explosives strapped to his body. Although his name was added to a terrorist “watch list” after his father expressed concern that his son might be a danger to the United States, the name Umar Farouk Abdulmutallab was not added to the “No Fly List.”

That oversight might have proved catastrophic, if not for the quick action of fellow passengers and crew members. The failure to prevent Abdulmutallab from getting on the plane in the first place has led to a flurry of new security measures—and no doubt some real consternation among some people named Johnson who find themselves on the list and thwarted at the airport.

A number of Robert Johnsons have been stopped by security in the years since 9/11, all because a man using the alias “Bobby Johnson” was convicted of a 1991 Toronto bomb plot. According to the journal Homeland Security Affairs, the US Government spends $100 million a year maintaining its No Fly List, intended to prevent known and suspected terrorists from flying to and from the United States. Along with the names you would expect, like “Osama bin Laden,” the list contains many common monikers—as numerous innocent Gary Smiths, John Williamses, and Robert Johnsons can tell you.

As of March 2009, several secret lists designed to prevent suspected terrorists from boarding airplanes contained over one million names. When aliases and duplications are taken into account, around 400,000 individuals holding airplane tickets run the risk of having their travel plans derailed as soon as they hit security. Only 5 percent are US citizens, however, and Homeland Security has asserted that the effective numbers are significantly lower than some reports have suggested. The odds a US citizen is on the No Fly List are 1 in 960,800, while the odds any person is on the list are 1 in 2,677,000.

If those odds seem low relative to the sheer numbers reported in the media, it's because the term “No Fly List” is often misunderstood as referring to the entire database of names and aliases compiled by the FBI over the years. The actual No Fly List actually prohibits about 2,500 people from air travel to or from (sometimes even over) the United States; a separate "selectee" list flags a larger number—some 16,000 as of October 2008—for extra scrutiny but does not prevent them from flying.

Despite the low odds of an individual being on the list, it has often made the news, thanks to inefficiencies like the namesake problem, which has led to absurdities like the 2004 flagging of Senator Ted Kennedy, the detention of a five-year-old boy in 2008, and the presence of the September 11 hijackers' names on the list years after they died in the attacks. All hope is not lost for those mistakenly targeted, however. The Transportation Security Administration (an arm of the Homeland Security Department) has a Redress Inquiry Program for people who feel they've been wrongly flagged, while a 2008 court decision upheld the right of passengers to challenge their inclusion on the list. The government has also made efforts to improve the system, such as “positive-flagging” those once stopped and cleared, and removing some mistaken names.

As we have just been reminded, no system dealing with such big numbers and so many variables (names, spelling, birth dates, etc.) is ever likely to be perfect, although the president and a number of lawmakers are pledging improvements in the wake of the terrorist attempt on the Northwest flight bound for Detroit. As it stands today, definitive proof that the existence of the No Fly List has improved homeland security is hard to come by. Who’s on the list—and others like it—remains a secret.