My experience is primarily with the two major aikikai-affiliated groups in the US (USAF and ASU). I've seen many dan tests (and a few kyu tests) over the years in both groups and have never, to the best of my knowledge, seen anyone fail a test.
I have seen this as well in another organization. I've heard the explanation that when a poor test happens it's the sensei that gets in trouble from the shihan doing the test, not the student. So the student passes, but the sensei gets lambasted who in turn is supposed to take the criticism and do more to prepare the next batch of testers. It kind of makes sense, except for the fact that sometimes people pass even if their test is horrible. Personally, I would rather flunk it and get a chance to do a better job. Spending years at a rank that I felt I really hadn't earned would feel rather awful.