This guy shot the pilots:
http://murderpedia.org/male.B/b/burke-david.htm
This guy stabbed one pilot to death before he was subdued:
http://murderpedia.org/male.N/n/nishizawa-yuji.htm
The stabbing occurred after he reported the security flaw he used to get the knife on board the plane. Previously checked baggage could be picked up after passing through security for another flight. He was able to simply take the knife out of a bag he had checked on a previous flight.
I'm guessing the number of weapons on U.S. planes that get by security run into the thousands every year. And before all the checks, probably the tens or hundreds of thousands per year.
Looks extremely rare. I mean really, really extremely rare. Handful of examples and roughly 9 million U.S. airline flights last year[1]
It'd be interesting to run some comparative statistics. I bet the odds show that you're safer with a plane full of gun owners, having their guns in holsters and loaded, than you are spending the same amount of time in each of those gun owners' homes.
By reporting this stuff they're actually making the case of how totally useless they are -- and how pointless and misguided the "find the weapon" school of security theater is.