The festival. The attendants don't respect the Literature Club, the whole group is heartbroken afterwards, and Natsuki, consumed with anger, tells Monika that Sayori would make a better Club President than she would.
Every single bad ending. Every single one. MC crosses the Despair Event Horizon upon Natsuki's and Sayori's deaths, the Literature Club disbanding leaves him without a friend in the world, and Yuri's view of herself bleeding out is both tragic and disturbing.
If you pass up both chances to go to therapy with Sayori, your relationship unravels as she gives up on MC by the time he realizes his mistake...
Natsuki accidentally breaks Yuri's tea set in the former's route, resulting in Yuri's uncharacteristic resentment for how expensive it was. Knowing that Yuri's parents are dead and her tea set was likely some sort of Tragic Keepsake doesn't help matters.
