According to a recent post on The TSA Blog, at least two passengers a day are caught at security checkpoints with a gun in their carry-on luggage. According to the post, when the passengers are caught, the most common response is "I didn't know it was in the bag." Unfortunately, that excuse works for the TSA just about as well as "the dog ate my homework" works for a high school teacher.
Once caught, a passenger potentially faces a number of consequences. First, the subsequent interaction with and interrogation by local law enforcement will quite often result in the passenger missing his or her flight. Next, the passenger could face criminal prosecution for violation of 49 C.F.R. 1540.111 which prohibits carriage of a weapon on your person or accessible carry-on luggage if security screening was required before boarding of the aircraft. The passenger may also be prosecuted under other local statutes that prohibit possession of a handgun at a checkpoint or in the secured area of an airport. [more]
Finally, and in addition to criminal prosecution, the TSA could also initiate a civil penalty action seeking to impose a civil penalty/monetary fine against the passenger for violation of 49 C.F.R. 1540.111 The penalty could range in amount from $1,500 to $7,500, depending upon whether or not the handgun was loaded. The civil penalty action is similar to an FAA enforcement action and does not provide as many constitutional rights and protections as a passenger would have in a criminal proceeding.
The TSA recommends, and I concur, that all passengers double check their carry-on baggage BEFORE arriving at the security checkpoint to confirm that they do not have a handgun or other prohibited item(s) in their luggage. Seems like a "no-brainer" to me. But, if you are caught "packing" at a checkpoint or in the secured area of an airport, hire an aviation attorney to help protect your rights.
For more information on the restrictions placed upon firearms at airports and in aircraft, please read my article on the topic: Carrying Firearms On Aircraft.