I have seen NOFORN printed in all seriousness at the top of a report cover. Apparently that is pentagon-speak for "no foreign distribution".

About a decade ago I went with cash in hand to several data recovery companies and asked them to recover some wiped files on a working disk. Distructive techniques were allowed, and the budget would have paid for some elaborate processes. Several first-level telephone answering folks said "sure, we can do it", but when I told the same story to the next level no one was willing to try. There is, of course, that old report claiming that there is enough information physically present in a multiply erased bit position to permit recovery. Maybe the intelligence agencies can do it, but I have heard of no verified case where they - or anyone else - actually did.