Each of the Mane Six except Rarity has had a character collapse at this point–that is, an episode where extreme circumstances lead the character to invert their own usual role on the show, revealing inner contradictions or hidden deaths.
Applejack had hers first in “Applebucking Season.” Exhaustion caused the normally dependable pony to become a danger.
Pinkie Pie’s is “Party of One.” Apparent abandonment leads the happy-go-lucky party pony to turn depressive, paranoid, and delusional.
Twilight’s is “Lesson Zero.” The stress of imminent failure makes the town’s best organizer into a force of chaos.
Rainbow Dash’s is “Read It and Weep.” Forced to sit still for a while, the bluntly realistic pony immerses herself in fantasy.
Finally, Fluttershy’s is “Putting Your Hoof Down.” As a result of assertiveness training, the kindest and most timid of ponies flips over into cruelty and pettiness.
Rarity has come close a few times, but never had a true character collapse–which suggests she’s next.