This question: Can you castle twice in Chess960? shows that there is a bug with the board playback regarding Chess 960 alternate starting positions, particularly in relation to castling. If you scroll through the moves, and then scroll backwards to before the castle move, the King ends up in the standard starting position rather than the modified 960 position, and if you keep going the Bishop ends up on top of him, scrolling forward again causes the King to move like a Bishop, and later on the Bishop performs a castle move instead of the King. You have to refresh the page in order to fix it. Not sure how difficult it would be to add a check when reversing a castling move to make sure the King ends up back where he started if it's not the standard layout.