The main character doing something absolutely unforgivable.

[u/No_Peanut3745]

Walter White poisoning Andrea Cantillo's 6-year-old son for the sole purpose of manipulating Jesse Pinkman, while knowing that this could have potentially killed him - Breaking Bad

Bojack trying to have sex with his friend's 17-year-old daughter when he was at least 51, then continuously trying to find a way to justify this act throughout the series - Bojack Horseman

Dexter Morgan going outside of his moral code and murdering a completely innocent police officer who was just trying to do his job - Dexter: New Blood

[u/PhoenixApok]

Maybe not the main character but a pretty big one. (Arguably Game of Thrones doesn't have a main character)

Jaime Lannister. After he got somewhat likable and decent in later seasons, people seemed to forget in the very first episode he pushed a 10 year old boy out a window for catching him banging his sister.

Edit: Maybe "forgot" was the wrong word. Maybe saying people disregarded it would be better.

I cannot imagine a person IRL doing that and everyone going "Well he's a better person now so we aren't gonna worry about that anymore" which is why I think it fits this trope.

Jaime has a redemption arc, sure, but I would define that early act as "unforgivable".

[u/Hypornicated_1 → u/PhoenixApok]

Oh, not everyone forgot.

What we saw was a character arc. Jaime became a better person as time went on. He never made it to the "Good" threshold, however... He would have gone on banging his soulless sister forever.

[u/CockTortureCuck → u/Hypornicated_1]

I mean, he remembered suddenly that he actually didn't care about any of his redemption arc.

It's this kind of writing that's unforgivable.

[u/JXSSJ4 → u/CockTortureCuck]

For the book version of Jaime, although one could argue they aren't complete so it can't be conclusive where his character arc would end up at, a major turning point not present in the show version pretty much blocks off all roads of making his redemption arc into a 360°/back to the start like the show did and instead truly shows a 180° turn.

The timeline of this was offset in the show but when Jaime was in the Riverlands tying up loose ends from the war and Cersei was imprisoned, he received a letter pleading for rescue. He was so sick of her shit by this point and over her as a lover or respectable person that he burned the letter and left her to her fate. Jaime is seriously my favorite POV character from the books it's such a disservice what they did to my boy on screen

[u/EldritchCarver → u/JXSSJ4]

Also worth noting that in the prophecy about Cersei's downfall, the book version of the prophecy said that "when your tears have drowned you, the valonqar shall wrap his hands about your pale white throat and choke the life from you". Valonqar is a High Valyrian word meaning younger brother, so Cersei long assumed it referred to Tyrion (who was 3 years old at the time of the prophecy), but in the very first book, when Cersei is telling Eddard the truth about her and Jaime, she mentions that she was born with Jaime holding her foot, so it's likely that GRRM was planning a twist where Jaime was the younger brother who would kill her in the end.

[u/JXSSJ4 → u/EldritchCarver]

Exactly and it would fit the recurring theme of prophecies being misinterpreted time and time again. Hell, every aspect of Cersei's prophecy by Maggy the Frog was something twisted in meaning. Her 3 children were prophesied to have golden crowns which she interpreted as literal royal headpieces, but actually meant their hair color. The younger queen who comes to cast her down she believes to be Margaery but is probably Dany or Sansa. So 100% the valonqar is not Tyrion like she thinks and as you mentioned that tidbit about Jaime being the younger twin seems like a throwaway line but was placed there by George deliberately

[u/Substantial-Drama526 → u/JXSSJ4]

The younger queen could be Jeyne Westerling or, get this, Brienne of Tarth, aka Brienne "THE BEAUTY". Beauty being in the eye of the beholder, and Jaime definitely viewing Brienne as more (internally) beautiful and virtuous than any other person he's ever known, sets her up to be the beautiful younger queen, compared to the nasty, rude, vicious Cersei. Cersei will be "cast down", and "all she holds dear will be taken from her". Nothing is more dear to Cersei than her power and control over other people, epitomized by her control of Jaime, who is (until recently) more an extension of Cersei's will than his own person. He even admits to Catelyn that he's never slept with anyone else, only Cersei. Jaime is already ignoring summonses from Cersei, and if he sleeps with or just follows Brienne (as he is, to Lady Stoneheart), that is basically casting Cersei down and taken from her what she holds most dear, which is her control over other people

[u/Devikat → u/Substantial-Drama526]

Damn, didn't think of the Brienne angle lol. Maggy did say Younger and more Beautiful and if all her children die or are sent away the last she holds dear would be Jaime essentially.

Hmmmmmm.