I can't vouch for the truth of it but several years back on a coin email listserver I'm on one member reported that he received his proof set order and inside one of the proof set boxes was the set along with a note written by someone who was also a member of that same email list. The note was one he had included with the set when he returned it that listed the problems with the set.
That would be an interesting test for someone on CT who receives a defective product from the Mint to do. Insert/hide a note that describes the purpose of the test (with some specific info only few would know) and that directs the person to the CT site with instructions to post that the set was re-purchased (even though was defective). TC