In Board setup 6 and Castling from /learn, the player's pieces are unable to move after capturing the opponent's king, thus the player can get stuck in the exercise without receiving any information about it.

For "Board setup 6", this happens when the capturing piece is defended by another piece:

For "Castling", the king capturing piece doesn't need to be defended, it only needs to be defended when capturing the piece in front/next to the king: