Espionage agencies have found peer-to-peer file sharing systems (like Kazaa or Bearshare) a steady source of useful secrets. An example of this is the recent announcement that detailed technical documents about the U.S. presidents helicopter (Marine One) had found their way to Iran, via a file sharing system. Apparently, an employee of a defense contractor had installed a peer-to-peer program on his PC. This, in itself, should not have been a problem. Peer-to-peer programs, during the installation process, ask you to identify which folder, or folders, on your hard drive, will contain the files you wish to share. But it's common for users to specify a folder used for other things (like holding files downloaded with incoming email). This means that anyone else on the peer-to-peer network can find any file in your designated "share" folder, and download it. Thus the Iranians go possession of the Marine One data files.
Unfortunately for spies, this type of peer-to-peer software is losing ground to Bittorrent type peer-to-peer software. Here, you have to specify a particular file to share, and use a rather more complex process (creating a torrent file) to put a file of your own out there for all to share. Nevertheless, spies are still having a good time with the older peer-to-peer networks, and much good stuff is obtained.