A flashback at the very end of the film reveals that Eun-ju witnessed Su-yeon being crushed by the cupboard, but did nothing to help her. This is even worse considering Eun-ju was going back to help Su-yeon, but decided not to just to spite Su-mi. Yikes.
Also, the sequence in which Eun-ju locks Su-yeon inside the cupboard that terrifies the latter so much and refuses to let her out, despite her heartbreaking screams. It turns out to be a subversion once it's revealed to be in Su-mi's mind.
The scene in which the wife of the girls' uncle vomits whilst having a fit (and possibly being possessed), before getting it in her hair and on her face during her spasms.
The fish carcass in the fridge.
Rewatch Bonus: Several scenes. The film usually requires a second or a third viewing to understand just what is going on, and, after the numerous revelations, almost every scene takes on a hugely different light.
What an Idiot!: A lot of the things Moo-hyeon does around his mentally ill daughter are immensely stupid, such as telling Su-mi to simply not talk about the wardrobe (rather than get rid of the obvious reminder of a horrible trauma) and leaving her alone when she's having a breakdown, and only serve to exacerbate the situation. May cross over with Values Dissonance.