When the defender's knight occupies the corner square itself, it blocks its own king and can be dominated. The wrong-coloured rook pawn suddenly wins because the attacking king seizes the key squares.
A famous exception to the wrong-bishop rule. Black's knight sits on h8, the very square the king needs, and the king is on f8 outside the corner. White has just played the one precise move Kf6!, taking f7 and g6 away from the knight; any other king move would have let the knight out and the king into the corner for the standard draw.