HOUSTON --TSA checkpoints at airports are at the front lines of preventing terrorism. When you go through security, you expect to be scanned and searched. And you expect TSA to prevent contraband from getting on planes, but as we've learned, that doesn't always happen. Houston businessman Farid Seif says it was a startling discovery. He didn't intend to bring a loaded gun on a flight out of Houston and can't understand how TSA screeners didn't catch it. Nearing the height of last year's Christmas travel season, TSA screeners at Bush Intercontinental Airport somehow missed a loaded pistol, one that was tucked away inside a carry-on computer bag. "I mean, this is not a small gun," Seif said. "It's a .40 caliber gun." Seif says it was an accident which he didn't realize until he arrived at his destination. He says he carries the glock for protection but forgot to remove it from his bag. He reported the incident as soon as he landed, shocked at the security lapse. "There's nothing else in there. How can you miss it? You cannot miss it," Seif said. Authorities tell ABC News the incident is not uncommon, but how often it occurs is a closely guarded government secret. Experts say every year since the September 11 attacks, federal agencies have conducted random, covert tests of airport security. A person briefed on the latest tests tells ABC News the failure rate approaches 70 percent at some major airports. Two weeks ago, TSA's new director said every test gun, bomb part or knife got past screeners at some airports. "It's very concerning. I'm very scared. First of al, I can't even believe it could happen," traveler Joy Mansfield said. "It makes you wonder what exactly all the security hoopla is all about if a loaded gun can go through," traveler Leeza Erfesoglou said. KTRK's Aviation Security Expert Jim Conway says screeners have a demanding job and are susceptible to fatigue, staring for hours at monitors while looking for prohibited items. "Look, this is simply human error," Conway said. "When something like this happens, it's human error. I mean, these folks are doing the best job they can." Seif and others say that's not good enough, not when lives are on the line. A representative for the Houston Airport System would not comment on the security breach at Bush Intercontinental Airport. A TSA spokesperson says the agency has conducted an investigation, saying remedial training was provided to the security officers involved in the incident. Advanced imaging technology and more stringent pat downs have also since been implemented.