Just saw this article on zdnet which talks about how the US Army kind of mucked up when it released a confidential report but didn’t convert it to PDF properly. So the censor’s pen wasn’t quite black enough – easier to see than holding it up against the light, I guess.
Of relevance to those of us not in the US military though – never (well, almost never) send a document to a client or external party with track changes on or with dodgy metadata in it (you’ll see dodgy metadata under /file/properties in your Microsoft documents – if you do that and it looks like something you don’t want a client to see, then change your templates). Better yet, do what I do and send them as PDF documents (try CutePDF) – just be better at that than the US Army is.