How do you send messages over the Internet when one of the most powerful spy agencies in the world is watching you? The answer is spam. After 9/11, the Taliban tried to hide messages from NSA surveillance by disguising them as spam emails, and it worked, at least for a little while.
By writing messages with spam-like subject lines, combatants were attempting to exploit surveillance filters so that instead of being combed, the messages would be sorted into the spam folder abyss.
Usually getting sent to the spam folder is a bad thing, but it's exactly what the terrorists wanted. One message that Taliban combatants sent had the headline "CONSOLIDATE YOUR DEBT." It looks like the same generic headline that could get an email sent to the spam folder in your Gmail account worked against the NSA's algorithms.
The U.S. ended up finding out about the tactic when it recovered some enemy computers in Afghanistan and took a look at the email messages it contained. They found that beneath the spam-like headline about debt consolidation, there was a message between two real combatants.
From a surveillance perspective, [cryptologist and former NSA officer Michael] Wertheimer writes that this highlights the importance of filtering algorithms. Implementing them makes parsing huge amounts of data easier, but it also presents opportunities for someone with a secret to figure out what type of information is being tossed out and exploit the loophole.
Wertheimer mentioned the tactic in his paper "Encryption and the NSA Role in International Standards."