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Tear Jerker: Pages Of Harmony
When Pinkie Pie gets her Element extracted in Chapter 3, her mane and tail go straight and her skin gets darker like what happened in Party of One. Seeing as this only happens in-universe when she feels that her friends have abandoned her, it really says something as to how much betrayal she feels toward Twilight.
Near the end of Chapter 6, Sweetie Belle begs and pleads for her sister Rarity to help her. By now however,Rarity has had the Element of Generosity extracted from her and her empathy and compassion for others completely destroyed. And the heartbreak in Sweetie Belle's eyes as she realizes her sister no longer cares about her just makes the whole thing worse.