And let's not forget "employee with a conscience", like that bastard Snowden.
Remember, he was actually quite careful about what to reveal. Nothing he exposed really endangered anyone, nor threatened the security. I'd be more concerned about guys with actual malicious intent, or simply dumber.
Now, while I would expect NSA to have such "legendary" security, it does indeed employ people. People who set passwords to "0000" or "swordfish", people who forget to log out or, perhaps, people who are interested in stealing data themselves. Heck, even simply nice, polite people who, upon being told that "Hey, I'm new here, could you let me in? Some bastard stole my access card on the bus." would simply open the door instead of calling security. While they probably are aware of possibility of all that happening, this is still putting all our eggs in one basket.