- The works of William Shakespeare have their own page.
- In Bat Boy: The Musical, Dr. Parker is a pretty sympathetic character until he murders a kid in order to frame the title character. After that, he's more of a monster than Bat Boy ever was.
- The Crucible: Abigail Williams crosses the line by accusing her close friend Mary of witchcraft to save her lies from exposure.
- In Dog Sees God, the character Matt is a homophobic bully throughout, his main target being the gay musician Beethoven, but he fully crosses this when he breaks Beethoven's hands so badly that he can never play the piano again. This is what causes Beethoven to commit suicide.
- In Giselle, it appears that the curse of the wilis only stays in effect if they had already danced a man to death; considering that Giselle's refusal to let Albrecht die on her first night with the wilis causes their spell over her to be broken and allows her to pass on, it's pretty clear that the other wilis have already crossed it before the start of the story. God only knows how many men they had murdered before they intercepted Hilarion...
- Hera crosses it in Herakles Mad when she fills the title character, who has just defeated a tyrant, with a homicidal rage simply because he's one of Zeus' bar sinisters.
- Regina in The Little Foxes is greedy and morally bankrupt as it is, but she crosses the event horizon when she lets her husband Horace die of a heart attack because he won't go along with her scheme.
- Medea committed numerous atrocities before the play even begins - although these were not all part of her story until after Euripides wrote the play. These include: a) chopping up her younger brother and tossed the pieces into the ocean so that her father would have to delay his pursuit to gather the pieces for a proper burial (this was so awful that Jason's intervention was the only thing keeping the rest of the Argonauts from tossing her overboard too), and b) convincing two kids to cut up their father and put the pieces in boiling water, under the pretense that it will make him younger.
- In the play itself, Jason may well cross by abandoning and betraying Medea after she sacrificed everything to be with him, effectively condemning her two children to a bleak future and possible servitude.
- If the aforementioned atrocities are not taken into account, Medea crosses during the play itself by killing her two children - depending perhaps on which of her motivations (Mercy Kill vs. hurting Jason) is given more emphasis. This is after she kills his new bride by lighting her on fire with magic poison.
- The eponymous man in The Phantom of the Opera crosses the MEH when he ties Raoul to a noose and forces a Sadistic Choice on Christine; either confessing love for the Phantom and buying Raoul's freedom, or confessing love for Raoul and watching him die. Lampshaded in Christine's song lyrics: "The tears I might have shed for your dark fate, grow cold and turn to tears of hate!" Though she still pities him by the end anyway, so maybe this doesn't qualify.
- Seneca's play Thyestes is about King Atreus' Moral Event Horizon. Mad at his brother, the title character, for stealing his wife and attempting to steal the throne, he pretends to call Thyestes and sons back from exile and serves Thyestes his own sons for dinner and takes great pleasure in telling him "you ate your children".
- Stanley's crossing of this in A Streetcar Named Desire came when he raped Blanche to insanity and then lied that he never once touched her afterwards.
- Caldwell B. Cladwell crosses this when he orders Bobby Strong to be sent to Urinetown, which is actually being thrown off Cladwell Column, knowing full well that his daughter will probably be killed by the rebels. Necessarily Evil or not, that was just unpardonable
- In Wicked, at least as far as Elphaba was concerned, the Wizard crossed the horizon when he broke Professor Dillamond's will, turning him from a respected professor into a mindless animal. For Mrs. Morrible, the horizon mark came when she created the cyclone that brought Dorothy to Oz(and killed Elphaba's sister Nessarosa). Interestingly enough, once Glinda takes the reins of government, she is willing to simply exile the Wizard from Oz (this might be because he's realized — to his horror — that Elphaba, whom everyone believes is dead, was his daughter), but has Morrible sent to prison.
- Greek tragedy has a term for this event: Harmatia. It's the act the Tragic Hero performs, usually motivated by his "hubris" (Tragic Flaw), that starts the tragedy down the path of no return. Normally this is some kind of offense against the gods or against the natural order: a murder, an act of blasphemy, a rashly-taken oath, etc. But once committed, it can't be undone. The Tragic Hero and every character around him is doomed, no matter how hard they try to escape their fate.
- Peer Gynt plays with the trope. Although it seems Peer Gynt himself is about to cross the Moral Event Horizon a number of times, he secures himself by hovering juusst over the edge of it. He even lampshades this as his leading principle. It occurs to him, a little late in life, that this constant playing with the trope gains him a Fate Worse than Death, and he crosses the Despair Event Horizon instead. And this freaks the living batshit out of him.
Moral Event Horizon / Theatre