These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Moral Event Horizon - 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.
For some, Su-mi crosses this when it is revealed that she snapped Eun-ju's bird's neck. Granted, she is crazy and delusional, and she is most likely doing it out of her hatred for Eun-ju, but, still. Poor bird didn't deserve that.
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. Of course, this trope is somewhat subverted once you realise that this was all in Su-mi's head.
Nausea Fuel: 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.