The feeling of betrayal, when Shana found out, that Mark has been hiding cameras in her house.
Alice's reaction to the death was, for some, even more agonizing than the death itself.
That whole scene was terrible. Alice is in a gift shop, and she finds a little plushie of a potted daisy that sings "You Are My Sunshine." Any genre savvy troper knows that this song is bad news in works of fiction. Alice buys the plushie for Dana, and goes to the hospital to see her. When she sees that Dana has just died, Alice slumps over onto the floor and just bawls and wails loudly, her face contorted in horrible agony. She drops the little plushie and it sings that song.
The fact that a gif of the following scene has reached Memetic Mutation-status on Tumblr actually kind of makes it worse, in a way...