I have had that happen occasionally on the Cycle ramp, with just the right crack. In fact, if you have any little physical cracks in the cycle ramp, that can cause it to flex just enough to allow the ball to flip up and over, once every 100 shots. I have never had that happen on my Comet ramp, however. If you wiggle the ramps with your fingers, and there is any play or movement in them, you would lose the ball. Should be quite rigid.
Like Vid says, the tiniest bit of height difference can make that happen on the Comet ramp also.