Yep. The main problem with transmitting human-based information, using humans, is that humans are lazy creatures of habit, and will do things that are.. well.. human.
A good example: A security firm was hired to test the security at a major corporation. There would be a file, located in the secure database, which they had to access. So they left a couple of flash drives laying around the area. Invariably, *someone* picked one up, and popped it into the computer (which was logged in) to see what was on it… Yeah.
I saw a PBS special on the Enigma machines, and how they were cracked, etc. Turns out a lot of the brute-force calculations and whatall were done by young women, to the point that the german commanders handed down an order that certain naughty four-letter words were NOT to be used as the code indexes, as they knew there were young women working the codesand that would be improper!
As for codes, try this one on for size:

It’s from the game Kingdom of Loathing (which I play). It was created by the devs to test just how freakishly smart their playerbase is, as well as how fast it would take the information to propagate through the game. The reward is a familiar that was awesomely powerful, but decreased in mojo with every new person who had one…
Acci: I’d have been tempted to include references to ‘Terminator robots’ and ‘orbital lasers’, as well as mentioning that certain enemy units had turned, and were providing information…