If the bearing still span freely then the bearing must of been broken on assembly as if it span freely there is very little load at all on the pin.

If the bearing broke up or the pin (somehow) managed to get from out side of the bearing to the inside then yes it could make its way into the crank case and then up into the cylinder. I Cant see how that could ever happen though.
_________________
If I have to take the carbs off once more...