Back in '74, we has a Comanche 260 in and while putting her in the hangar, I noticed a ding in the stabilator. It had been filled and faired, but I thought to check... Nothing in the logbook, so checked the static balance...It was out by a bit, so corrected that with ballance weights and passed it.
Thing was, not long before, a similar a/c had the same problem and it rang a bell. In that case, the test flight was done by the owner. To make the weight, he got some friends along...Doing the Vne part of the test, he stuck the nose down and reached the Vne while still in the dive, rather than gently pull up and reaching it in level flight.. Result was, he exceeded the Vne and the stabilator fluttered and departed. Sudden pitch down, wings folded and that was it.
I think that was the case that resulted in all C of A test flights to carried out by comercial rated pilots. The investigation found that the stabilator was out of balance due to 'hanger rash' and had been repaired without thinking it through.