Misuse of the "Hard" button is especially problematic for FSRS, it assumes that if the user forgot the card, he will press "Again".
I propose two design changes:
1) Make the distance between "Again" and "Hard" noticeably greater than the distance between "Hard", "Good", and "Easy".
2) Give them colored borders (or backgrounds) in such a way that "Again" will have a different border/background than other buttons.
Do you like this design?
Subjectively; irrespective of whether you think it's helpful or not.