George R.R. Martin has confirmed the three biggest spoilers he told Game of Thrones showrunners David Benioff and D.B. Weiss when it became clear the show would overtake his A Song of Ice and Fire books. Game of Thrones started in 2011, the same year that Martin's fifth novel in the ASOIAF series, A Dance with Dragons, was published. With several years of material for the show to get through and Martin planning on two more books, he initially felt confident that he could finish his story before the show caught up with him.
Within a few seasons, though, it became clear that wasn't the case. Not only was Game of Thrones making some big changes to the books, massively streamlining storylines that likely would've necessitated it going beyond the eight seasons it eventually ran for (which Martin was counting on), but the author himself was struggling to write The Winds of Winter, which still doesn't have a release date. Because of that, Martin sat down with Benioff and Weiss for a meeting in 2013, after season 3 had aired, to go over his plans and where the story would end.
The now-fabled meeting saw Martin divulge what were described as three "holy s**t" moments from the books, all of which made it into the show, including one said to be from the very end. After they happened in Game of Thrones seasons 5 and 6, respectively, it was confirmed that Stannis Baratheon burning his daughter Shireen was one such moment, and that another was Hodor's death while holding the door. In James Hibberd's book Fire Cannot Kill A Dragon: Game of Thrones and the Official Untold Story of the Epic Series, Martin again confirms these two moments are from his upcoming books, alongside Bran sitting the Iron Throne at the end. Martin said:
"It wasn't easy for me. I didn't want to give away my books. It's not easy to talk about the end of my books. Every character has a different end. I told them who would be on the Iron Throne, and I told them some big twists like Hodor and 'hold the door,' and Stannis's decision to burn his daughter. We didn't get to everybody by any means. Especially the minor characters, who mau have very different endings."
It had previously been confirmed that Bran becoming King was from the books, but it wasn't clear whether or not that was among the spoilers the showrunners had previously teased. Since it was said to be from the end, there had been speculation that it may have been regarding Daenerys Targaryen instead - either her decision to burn King's Landing, or Dany being killed by Jon Snow in the Game of Thrones series finale. Both of those moments, which fit the above criteria of being suitably shocking, are presumably from the books, especially with Daenerys looking increasingly likely to engage in a Dance of the Dragons-style showdown with Aegon Targaryen (the one the show didn't include), but Martin's comments here seemingly confirm that the third "holy s**t" moment was indeed Bran on the Iron Throne.
How all of these events happened on screen isn't necessarily how they'll go down on paper. In the same book, Hibberd notes that Shireen's death will "play out a bit differently" in A Song of Ice and Fire, while Martin himself directly confirms that Hodor's death will be different in the novels too, as he'll be wielding a sword rather than literally holding a door. Based on that, it will also mean the circumstances leading to Bran becoming King are quite different too - and likely involves more exploration of him as the Three-Eyed Crow, as well as greater detail in the actual process of the Lords and Ladies of Westeros crowning him - but it once again re-iterates that, for all it's one of Game of Thrones' most controversial choices, it will be happening in the books as well.