Small bug: you cannot place a marking on the edge below the bottom right black ball.
It would be too much trouble to get this fixed (we have to wait for the author to fix it, which could take ages), and it does not affect the solving path at all, so I think we should merely keep it like this unless valvino states here that he would like to fix this.
Good eyes for spotting this bug. However, I agree that it is minor and doesn't change the puzzle that much at all. I'm happy for it to be left in on this occasion.