This is an old bug but I haven’t played with demo board in a while.
I was watching a demo board and stayed behind of the controller. I was just looking at the board position while the controller was continuing replaying the game and add stones to the game. I could see the stones appearing for a fraction of second in the board. Some sort of rendering issue.
I managed to record it with my phone… The file size is large so sorry about that
Edit: Using latest Chrome on Windows7