How it happens
This happened to a friend of mine about six months after 9/11, when everyone was still very jumpy.
He had a real grenade that had been rendered inert he kept on his desk as a souvenir from his time in Desert Storm. He was still in the National Guard at the time and thus had a military ID.
Unbeknownst to him, the night before an early morning flight, his son took the grenade off his desk and ended up putting it in his carry on bag he'd packed and set next to his desk. He went through security, they saw it on the Xray, and immediately hustled him into a room for further questioning. Fortunately for him the TSA guy was a former Desert Storm vet himself, could see the grenade was no longer operational, and bought his story, so he still made his flight on time. The grenade was confiscated, however (somehow I bet the TSA guy ended up taking it home)
Obviously he wouldn't have deliberately packed a grenade, live or otherwise, but sometimes strange things happen when you have kids. I'll bet he checks the contents of his carry on before leaving to this day :)