A friend of mine, who's a public relations guy in the firearms industry, had a carry-on duffle bag with a stiff plastic plate in the bottom. We made a trip to New Zealand together in 2007, flying of out of LAX, and when we returned the TSA people found a fired .30-30 case under the plastic plate. The interesting thing was that he hadn't taken a .30-30 to NZ, and in fact hadn't traveled with a .30-30 in around 2-3 years--and the LAX TSA folks hadn't found it when we checked in to fly over to NZ. (Luckily, he didn't have much trouble.)
Deer Lake CA (Newfoundland) - leaving! "there's a loose round in your bag" - OK, they make me tear the bag apart. I showed them my box of .300 Savage with all the rounds in it (since Air Canada had lost my rifle on the way up I ended up shooting my moose with a borrowed .30-06). Nope, run it through again. Still there. Finally under the little flap at the bottom of my day pack I find a .257 Roberts round. It had to have been 5 years since I'd carried that rifle deer hunting. In the meantime that pack had been a carry on to England, Germany, Turkey and Canada (on the way in 5 days before!) and numerous US airports.
