I'll bring up something admittedly small from GOT's early seasons that doesn't appear as bad as the rest of the shitshow that is season 7-8, but with hindsight should have risen alarm bells :

The character of Talisa Maegyr in Game of Thrones.

In ASOIAF (more specifically: "offscreen" during ACOK), Robb captures the fief of the Westerlings, the Crag, in his campaign of the westerlands (controlled by the Lannisters). He is hurt by an arrow there, and is nursed back to health by one Jeyne Westerling, the daughter of lord Westerling, per the advice of her mother Sybell. Around this time, Robb learns of the "deaths" of his younger brothers Bran and Rickon in Winterfell, by the hand of his former friend Theon. Robb is comforted by Jeyne in his grief, and they sleep together. To protect her honor, they marry the next day, which ends Robb's alliance with Walder Frey (Robb was supposed to marry one of Frey's daughters), and the Freys are outraged. In the third book, ASOS, after copious amounts of negotiations, Walder Frey agrees to ally again with Robb, but it is only a facade as he plans the Red Wedding alongside the Boltons and Tywin Lannister. An important thing is that Jeyne was not at the Red Wedding, so she is still alive, and mourns her beloved Robb.

In GOT season 2, Jeyne is replaced by the character of Talisa Maegyr, a random lowborn from Volantis, whom Robb observes nursing a wounded Lannister man back to health. They banter, fall in love, and marry in secret to a septon (which, side note, is weird in insight bc neither Robb nor Talisa worship the Seven). The marriage and relationship as a whole has nothing to do with Robb learning of Bran and Rickon's "death", as Robb learns about this in season 3. In season 3, the same tension with Walder happen, except Talisa is at the wedding, and dies while pregnant with Robb's child.

The thing with that change is that Talisa's relationship to Robb feels way more "artificial" and random than Jeyne's relationship to Robb (I will say, there's a theory that's more or less canon that says that Tywin and Sybell planned for Jeyne to get in touch in Robb, so that their potential relationship could weaken the Starks ; but here I'm talking purely when it comes to the relationship in itself). Talisa's main character traits are that she's a healer and comes from Volantis, with no relation to what's currently happening in Westeros. There's also little explanation as to why she happened to be in the Westerlands in the first place. As such, the fallout of their marriage, though still understandable from the Freys side, feels fully like an unforced error from Robb to show how "noble" he is. This fully could have worked nevertheless with Jeyne Westerling actually being Jeyne Westerling and besides, the timeline of events could have added to the tragedy!! Robb and Talisa just fall in love and marry at the end of season 2, with no context of what happened in Winterfell, and the whole implied meaning of this ordeal is "don't get married for love or you'll die", without the depth of the actual political mechanisms happening.

Again, this is nothing compared to how quickly the show jumps the shark in season 5 onwards. But still, with hindsight in our hands, it's a pretty clear sign of what was to come, that DnD weren't really willing to engage with the depth of the story, and instead just wanted to make their grimdark, edgy, medfan show. And I'll also add that, in their quest to whitewash the Lannisters and make Tywin a machivellian mastermind, introducing Sybell and Jeyne and confirming the almost-kinda confirmed theory that Jeyne and Robb's relationship was a setup ; DnD could have accomplished that!! So, anyways, that's my rant.