As part of a small project, I mocked up a "confidential" "NSA" document. While it isn't real, friends are saying that it's convincing enough that I should place some text at the bottom saying
Please note that this is only a replica, and in no way intends to masquerade as a real NSA document.
Should I add anything else, i.e. also saying "We (the creators) do not have the authority to create real NSA documents."?
Yes. You should put a disclaimer on the document or in a watermark. Government officials charged with enforcing national defense secrecy laws are not good at getting jokes and could easily assume it is real, and once you are accused, even wrongfully, of a national security offense related to classified documents, getting due process let alone a quick dismissal of the case, can be expensive and difficult, if it is possible at all.
If the point is just to have a fake classified document for an example or illustration, I'd put a fake organization name on it. Like don't make it the NSA, make it the MSA, and make your own logo for it.
I've made fake documents for examples in software user guides and that sort of thing. When I can I deliberately make them ridiculous so that no rational person would think they're real. Like if I was going to make a fake classified document, I wouldn't make the text be about some real international crisis or controversial political issue. I'd make it be about a zombie apocalypse or something of that sort. That way, most people should quickly realize it's deliberately fake, and if someone doesn't and actually tries to take you to court, you can make them look ridiculous.
