Have witnessed three, seen one, made one (not the one I saw; the one I made was beyond a cross-green ridge at the front of a false back, and I saw the ball disappear from sight ... but I'd seen that several times on that hole, and had always found the ball over the back).
My cousin Charlie's, at Sutton Bay No. 13 (a long drop-shot par-3) went right into the sun, and we all lost it. We knew it was a good shot -- but spent several minutes searching all around till we thought to look in the cup.