For a cute, saccharine show, My Little Pony: Friendship Is Magic has a lot of villains that cross the line, proving that even though Equestria is all about instant forgiveness there are still some things that are considered unforgivable.
- In A Canterlot Wedding, Imposter-Cadance/Queen Chrysalis crossed this by leaving Twilight trapped in the mines under Canterlot which were long forgotten so nopony would ever find her. Then taunting her with that fact, just to rub it in. And then it's revealed she did the exact same thing with the real Cadence, who is in rough shape by the time Twilight finds her. All this after letting an already-broken and regretful Twilight think all is forgiven. If not then, she crosses it in To Where and Back Again where she attempted to drain all of the love from Thorax for his betrayal just to further stroke her delusional leadership.
- The Crystal Empire does this to King Sombra immediately upon his introduction, in which it is revealed that he used Black Magic to turn the Crystal Empire into Mordor and enslave the crystal ponies who lived there. Worse still, he's given no excuse for his crimes - Nightmare Moon had a tragic backstory, and though it doesn't remotely excuse their actions, Discord and Chrysalis were at least motivated by some component of their natures. Sombra, though, was just a seemingly regular unicorn who went to seize power for himself even if it meant ruining the lives of everyone around him. Appropriately, the episode presents him more as a monstrous force of evil than as an actual character.
- In Twilight's Kingdom Part 2, if Lord Tirek hadn't crossed this already, he certainly did when he had Discord imprison Applejack, Rainbow Dash, Pinkie Pie, Fluttershy, Rarity and Spike so he can drain their magic, then betrayed Discord and drained him of his magic. Even earlier than that, he banishes Celestia, Luna, and Cadance to Tartarus, even though they have no magic and are powerless to do anything against him.
- Interestingly inverted with Discord. In "The Return of Harmony", he almost crossed it by mind raping the main cast. A few seasons later, after taking a level in kindness, you'd think betraying all of Equestria to Tirek, including Fluttershy, would count as this, but due to his recurring remorse and reaction to Tirek betraying him not only make him sympathetic, but solidify his HeelFace Turn.
- In My Little Pony: The Movie (2017), The Storm King might be silly, but he's responsible for a lot of suffering. However, the point where he crosses this line is either his enslavement of Canterlot's population (which is treated dead seriously) or stabbing Tempest in the back and attempting to outright murder her in cold blood when she asks for him to fix her horn to fulfil his end of the deal. He may have crossed this right before the latter by creating a massive tornado that threatened the lives of not only his uprising opponents and newly aquired slaves, but HIS OWN ARMY AS WELL!
- In the Season 8 finale, Cozy Glow crosses it big time when she imprisons the Main Six in Tartarus, takes over the School of Friendship, and attempts to take over Equestria. If that wasn't enough, then the way she does it certainly counts; she has all of Equestria's magic stolen so she can send it to another dimension, takes out a de-powered Starlight Glimmer so she's unable to stop her, does the same to Chancellor Neighsay while giving him a massive "The Reason You Suck" Speech, and when the Student Six try to stop the act, she turns the entire school on them and shows no concern for any of them when they get caught with Starlight and almost sent to the same dimension. Her actions were so horrific that she ended up in Tartarus with Tirek as punishment.