I have been through three piles of roughly 2,500 marbles and all of them were mint. I consider that sample and those amounts to be a miniscule representation of the overall production amount. I know that there are problem marbles that come out of every production - but given that production numbers for a run are said to be >250,000 - 300,000 - there would seem to be room to sort and dispose of problem marbles. And since there are contemporary glass workers that use problem marbles from these runs for their work - there is a good channel for less than mint/round mistakes.