GOD KNOWS we all make mistakes, but there's one phrase you probably should never utter at an airport-security checkpoint:

"Ohhh man, I forgot that was in there!"

(OK, also maybe a few others, like anything that involves cupping your hands around your mouth and yelling, "Hey everybody . . . !" But we digress.)

In the post-9/11 world, firearms and explosives obviously sit at the top of any list of items that can't be stuffed into your carry-on bag.

But statistics show that the number of firearms confiscated at airport-security checkpoints across the country has been climbing steadily for seven years, according to the Transportation Security Administration.

Last year, the agency discovered 2,212 guns - the vast majority loaded with live ammunition - in carry-on bags at 224 U.S. airports. Quite a jump from 2007, when the figure stood at 803 firearms.

Just 11 firearms were found at security checkpoints at Philadelphia International Airport - a far cry from the No. 1 spot held by Dallas/Fort Worth International, where a whopping 120 firearms were confiscated.

All told, in Philly last year the TSA collected 4,000 pounds of prohibited items - which ran the gamut from seemingly benign things, like bottles containing liquid and gels, to brass knuckles and knives.

What gives?

"It's definitely not proportional to the increase in passenger volume," TSA spokesman Michael McCarthy said.

"We don't know what it is. The numbers continue to trend upward, which is concerning to us.

"The vast majority of people say they simply forgot it was there . . . We're not necessarily seeing malicious intent there. It's more like absent-mindedness."

Sure, we live in an age of endless agita and distraction, of rushing from one thing to another, our necks and faces perpetually bent down toward our tablets and phones.

It might be easy to forget that you have, say, a mostly empty 12-ounce bottle of vending-machine water in your bag as you barrel toward that flight you can't miss. (Liquids in containers larger than 3.4 ounces aren't permitted through security checkpoints, according to Philly International's website.)

But could you really forget that you have a loaded folding-stock rifle - with two loaded magazines - like the one that a TSA worker found in a passenger's bag last year at Dallas/Fort Worth International?

Or the improvised explosive device training kit - complete with inert blasting caps, detonators, detonating cord and C-4 - discovered last year in a checked bag at Honolulu International Airport?

Or the dissembled .22-caliber found hidden inside a PlayStation 2 in a carry-on bag at New York's John F. Kennedy International Airport? The knives tucked inside a highlighter and pen at Philly International?

Philadelphia Police Chief Inspector Joe Sullivan, who oversees Homeland Security and Counter-Terrorism operations for the city, chalks up the rise of carry-on surprises in part to passengers' pure laziness.

"You can legally transport firearms. The TSA makes it pretty painless," he said. "But it's an extra step at the airport. People show up late, and they're just trying to take the gun where it's going."

McCarthy said a traveler who wants to transport a legally owned firearm has to declare the item to the airline and store the weapon in a locked hard-shell container. The container must be checked luggage that can be properly marked by airport employees.

The agency is expected to crack down even harder on rules and regulations in the wake of a recent Department of Homeland Security investigation involving undercover agents who successfully smuggled weapons and explosives in 67 out of 70 attempts at numerous airports.

But still, some travelers think they'll be able to simply slip between the cracks.

"The further we get out from an actual domestic terrorist incident, people become desensitized," Sullivan said. "They expect to see less security."

This year, six firearms have been found at checkpoints at Philly International.

Sullivan said the weapons tend to be legally owned. But having the proper paperwork won't save you from the time-devouring saga that unfolds when a weapon is found at a checkpoint.

"It causes a lot of disruption and alarm," he said. "We'll charge you with disorderly conduct and confiscate your permit."

Police officers have to be summoned to handle and unload the weapon, and the Police Department is required to notify the FBI and other agencies about the incident.

Confiscated firearms are processed at the department's Firearms Identification Unit and returned - eventually - to the state or county that issued the owner's permit.

"It's really hard for me to understand why somebody thinks that if they have a permit to carry, that grants them the ability to carry a firearm on an airplane," Sullivan said. "It's a common-sense issue. We're just trying to keep people safe."

On Twitter: @dgambacorta