This was my fourth game of Zertz, and it was the first game I played with any real seriousness. Binyamin and I thought things through in various directions, and made various exchanges. Each turn was carefully considered.
As the game rolled to a close, Binyamin needed either a white or gray stone to win, while I needed either a gray or black stone. And then we hit a situation not described in the rules.
This was surprising, since Kris writes his rules very well, and his games are designed with elegance. He even adds rules for special infrequent situations that may come up. Unfortunately, he missed this one.
In our game, Binyamin was able to place a stone but not remove a disk; all the disks were interior to the board. So, the balls were not isolated, but he was also unable to complete his turn. We didn't know if that meant that he should remove a disk, even though he couldn't slide it out, or if he didn't remove a disk at all. Of course, one way I would win, and the other way he would win.
The game was still an excellent game, aside from this quirk.
Update: Ah ha. I thought Kris wouldn't have overlooked this. From the rules:
However, it may occur that you cannot remove any of the vacant rings without disturbing the position of the other rings. In this case you must not remove a ring (i.e. your move ends after having placed a marble).