If your remaining fatigue is exactly the amount required to launch two attacks upon an enemy that is using riposte, the skill icon continues to appear active if you miss the first attack, and your fatigue pool displayed on the combat screen does not update to include the fatigue added via the riposte (i.e. it appears as though you still have enough fatigue to make another attack), but you are unable to use the skill again. If you enter the character screen, the added fatigue is accurately reflected there, and then it is also accurately reflected (and the skill icon is disabled) once you return to the combat screen.