Secrecy

Something that is fundamental to secret operations is not telling anyone what you are working on. What you are working on, and the material you are using does not need to be secret, but the fact that you are thinking about it is what you want to conceal. I suspect that might be the reason that most of the stuff that is coming out of Wikileaks looks like common knowledge: it's because it is common knowledge. The fact that someone was talking about it, that was the secret. Unclassifying information can be hazardous as well, because then people can put together what you were thinking about, which can give them clues as to what you might be thinking about now. At some point it just becomes ridiculous. If you really want to have secrets, don't write them down.