The treatment of Sam (and the Sam/Frodo friendship) in Return of the King movie, and the treatment of Faramir as a character. I kind of get what they were going for, it just went way too far overboard imo and probably shouldn't have even been attempted. I'm assuming a bit of knowledge in the unpacking of these below for brevity but hopefully it is not too unclear for those who may have only watched the movies.
In the books, Faramir was pretty much the one redeemable part of the house of stewards. Yes, the ring may be tempting, but I think that was already sufficiently shown throughout earlier moments in the movies and there was no need to throw Faramir under the bus. He was already a relatively minor character in the books, why do him dirty like that? Everyone, both the characters in the book and the readers outside the book, looked up to him. One of his men broke a rule (punishable by death) to save him in the books and from the context you can imagine most of his reports would have done the same, but if all you knew of him was what you saw in the movies, it gives a very different impression. He came around in the end but at the very least he should have done the right thing sooner than he did in the movies.
As far as the Sam (and Frodo) thing goes, admittedly it's been a year or two since the last time I read the trilogy and it's easy to confuse the books/movie in my memory, but I don't remember seeing any signs of contention between them the way it was shown in the movie, and certainly if there was, it was not even close to the same degree. Among other things, I think the movie was trying to show the darker side of the ring's effects, but again, it's already been shown multiple times throughout the movies and they just went overboard with it. Frodo pretty much had the highest trust for Sam (even in the movies this is basically true outside this incident), and would Sam really give up and abandon Frodo that easily because of a few crumbs? Tangentially, I don't remember Sam being particularly cruel to gollum in the books, and that's really the root of this breach of trust in the movies. That entire aspect of Sam shouldn't have been there to begin with, and if it wasn't, the staircase scene would also make no sense to show that way.
The reason I find these two particularly inexcusable is that I simply don't see how they added much to the movies in terms of story or character development. There's an argument for it but it's just weak imo, Tolkien had it figured out just fine thank you very much. I also don't see how it would have helped in terms of budget, reducing film length, etc. I can excuse other omissions or slightly weird things if there are budget/time constraints, you can't reasonably ask for the impossible, I just don't see it in those two cases.