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.
Hilarious in Hindsight: The prologue of Ace Combat: Wings of Unity has Princess Celestia tell Princess Luna that she had a vision regarding Aurora's return. Fast forward to the recent finale of Season 4, and Luna tells Celestia almost the exact same thing regarding Tirek's return.
The prologue was posted on July 2, 2013, almost one year ahead of the episode!
General Silverbeak appeared to have crossed it by suggesting a plan to kidnap little fillies and using them as hostages in the early phase of the war. It is revealed in chapter 14 that it was actually Red Cyclone's idea.
Gilda's betrayal and her brutal beating of Rainbow Dash in chapter 11.
Nightmare Fuel: The second chapter of Wings of Unity has the heroines discuss how Aurora managed to get her wings. She got them by combining magic and alchemy, or, to put it bluntly, using the blood and bones of her fellow Exiles, be it those who died or would die soon.
As awesome as the Princesses wiping out an Exile army with their magic in Chapter 8 was, their targets all died VERY painful deaths. There's a reason Celestia and Luna only get involved personally if there's no other choice. (Now, imagine that they are forced to turn that power on a city full of civilians...like the city the Exiles have built, for instance.)
The horrific, grim scenery and the sight of wounded or dying ponies is very much this for the Cutie Mark Crusaders in chapter 10b. They try to ignore it, but Dinky is still disturbed by it.
Especially since the Crusaders helped Firecracker Burst kill some Exiles themselves. One soldier from a tank they sabotaged moans in agony from horrible burns before dying... right in front of them.
Firecracker Burst's... enthusiasm for using her fire magic on the Exiles is rather unnerving, especially given how she laughs her head off among all the screaming.
The Woobie: Every main heroine at some point, but especially Derpy.
Cloud Kicker in chapter 15 of The Equestrian War as well.
Firefly had a really rough life to say the least.
Pearl Eyes in Wings of Unity; despite being one of the Exiles, she is definitely the most sympathetic of them so far.