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.
Literally everything Nightmare does is either this or a Kick the Dog moment.
Discord apparently crossed it in the backstory, when he killed Celestia and Luna's parents — he Mind Raped them so far past the Despair Event Horizon that they simply lost the will to live and wasted away.
Never Live It Down: In a humorous moment shortly before everything goes to hell, Twilight is shown worrying that this will happen in regards to her breakdown in "Lesson Zero".
Tearjerker: In real life, the author's untimely passing.