The moment when Shirai-sensei calls out Kuro's constant taunting of Aoki over being a virgin as cruel and wrong. It is the only time in the entire series when anyone has called her out.
Aoki declaring he will be supervising both Rin and Reiji because they are both too dependent and stubborn to properly care for themselves. What makes this awesome? He knows full well he's abusing his authority and challenges Reiji to do something about it. Add this little scene, and try to compare it with what happened in the anime's penultimate episode.