I always emphasize to my students that if you are carrying, you should ALWAYS know where the firearm is, what condition it is in, and NOT carry into a known prohibited place. We go over extensively how to legally fly with a firearm and what parts of airports are prohibited.
Had a colleague several years ago that accidentally left an empty magazine in his carry-on (firearm and ammunition were packed IAW TSA requirements). This was in VA, not TX. He was pulled out of line, temporarily handcuffed while they did a very complete search of him and his carry-on bag. He was released, but the magazine was confiscated. He received a very nasty letter from TSA, but no fine--however, was told if it happened again, there would be a hefty fine.
Check, check, and re-check your stuff before going to the airport and if travelling with a firearm, read and adhere to the "TSA Flying with a firearm" information.