Overhitting wasn't even in my considerations. It was a 2 foot uphill putt; I gave it a gentle tap, the ball went up to the cup as usual, disappeared as usual (registering the putt with the usual "click-click"). THEN, for no discernible reason, it reappeared the other side of the cup (as if it was standing still) - and accelerated, uphill and away to 13 feet from the hole.
I've seen balls reappear when you're given a "gimmee" (Your ball's right back there when the next player takes their shot!!!). I thought that might indicate a glitch. But the difference with "gimmees" is... the ball doesn't proceed to race away...costing you another shot.