Three well-documented incidents:

Richard Reid, who managed to get aboard with explosives in his shoe

A man who boarded a flight out of New Orleans in late 2001 and realized after the flight had taken off that he had boarded with a gun in his briefcase. (He owned it and had a permit; had forgotten to leave it at home.) "Security" never noticed.

The recent (within the last year) case of a student who got through "security" with various contraband and publicized it because he wanted to show that there were loopholes in our system. If he were my kid, I would have been proud of him.