Rei Asaka's apartment, which only has mirrors and Creepy Dolls aside of two/three furniture pieces. And this is without counting a certain spot in school, with the knife holes on the walls... Room Full of Crazy, indeed. Doubles as a Tear Jerker, considering the circumstances in which we find out about it.
Also, the scene in which Fukiko is swimming in a nearby pool and Nanako catches her. Fukiko invites her to take a dive, and it's even a little cute... and then she gets Nanako's feet stuck under a log and almost downs her. Of course she only intended to scare Nanako off, and gets her out in her arms when the poor girl is about to pass out, but it was still SO CREEPY.
The boxcutter deal came to a head in Episode 27, when Aya's bullying crosses a line too many and thus provoking Mariko to enter a Tranquil Fury and slash Aya with said boxcutter.
The scene where Rei, after being overloaded on drugs and with a high fever, mistakes Nanako for Fukiko and tries to kill her. And herself. Then turns into a Tear Jerker when Nanako barely manages to snap her out of it, and Rei collapses in tears.
While comparatively mild, after Nanako runs out of her birthday party, Mariko grows depressed and refuses to eat anything. Kaoru notices and tries to cheer her up, but Mariko angrily rejects her, saying "What good is health? Who cares about health!". The usually level-headed Kaoru becomes so infuriated that she lifts her into the air, shaking her and screaming into her face.