Invariably, according to the T.S.A., travelers at airports with guns in their carry-on bags say they simply forgot they had them. “It’s almost always inadvertent rather than intentional,” said David Castelveter, a spokesman for the agency

Like other professionals in security, law enforcement and firearms safety, Mr. Castelveter was baffled by how anyone could forget that they were carrying a gun. “I’m a Vietnam vet, and when I went through training I was taught that my gun was my best friend — and God forbid you should ever lose sight of that fact. I would never, ever not know that I have a gun in my bag.”

Yet that was the exactly the excuse offered by a 27-year-old flight attendant who was stopped at a checkpoint at the Philadelphia airport on Sunday. The flight attendant, arriving for work on a US Airways flight, had a valid handgun permit — but of course, not a permit to carry it on an airplane. As it routinely does in such cases, the T.S.A. notified local law enforcement. A Philadelphia police officer who responded tried to unload the 38-caliber handgun weapon but instead accidentally fired it. No one was hurt, and the flight attendant was issued a summary citation for disorderly conduct.

It could have seemed like a Keystone Kops episode. Instead, it occurred as air travel has become increasingly tense. The potential for trouble posed by prohibited guns on crowded airplanes is obvious, even beyond any overt issues of terrorism or premeditated crime.

Except in rare instances where T.S.A. officials believe the Federal Bureau of Investigation needs to be notified, local law enforcement officials usually handle reports of guns at airport checkpoints.