Recently, a U.S. Department of Defense web document was found to contain classified information that was supposed to be blacked out, but wasnt. The culprit in this case was the Adobe Acrobat program, its editing tools, and a military user that did not fully understand how to use Acrobat. This has happened before, not just with PDF (Acrobat) files, but also with Microsoft WORD files (where stuff needed for the undo function stay in the file unless you go to the effort to flush them out, which few people do.) 

Another source of leaks, is simply the ease with which electronic documents can be passed around. Although the military constantly stresses the need to be careful with classified information, they also stress the efficiency of electronic documents. Paper is pass in the military, with laptops a common sight even on battlefields, and email attachments are a favorite way to pass things along. Unfortunately, all a sender has to do is pass on a secret file to one wrong person (usually by mistake), and that document is in play (leaked all over the net.) Its actually worse than that, because a lot of supporting documents and digital pictures, which eventually become part of classified report, are often freely passed around first by troops who arent thinking too hard about security classifications. 

Another source of leaks is people putting lots of classified material on their laptops, which then get lost or stolen. Its unclear if this has actually resulted in the bad guys actually getting their hands on the sensitive material. Such stolen laptops are using sold as stolen laptops, not repositories of classified data.

Some of the most damaging leaks are done the old fashioned way, by having someone say the wrong thing at the wrong time. Politicians often do this, and frequently do it deliberately. But there are also a lot of accidental releases of valuable stuff by people who should know better. The most notable, and damaging, example was in the late 1990s, when civilians getting a guided tour of NSA (National Security Agency), were told by a staffer that the NSA was listening on Osama bin Ladens satellite phone conversations. That got into the news, and shortly thereafter, Osama stopped using his satellite phone. Sometimes you dont need a computer to screw things up faster.