*Warning: SPOILERS AHEAD!*
George R.R. Martin, author of the books on which the Game of Thrones TV show is based, was recently asked if he had a least favorite difference between the two mediums. He replied:
I think one of the biggest ones would probably be when they made the decision not to bring Catelyn Stark back as Lady Stoneheart. That was probably the first major diversion of the show from the books and, you know, I argued against that, and [David Benioff and Dan Weiss] made that decision.
In the books, Catelyn Stark returns to Westeros from death after being revived by Thoros of Mir. She roams the Riverlands as a vengeful wight, silent and partially decayed, seeking vengeance on the forces that left her family in ruins. It's pretty metal.
Many fans think they saw a nod to this alternate universe in the latest episode of Game of Thrones.
As one fan writes on Reddit:
If you watch the sparring scene in [season seven episode four, “The Spoils of War”] between Arya and Brienne, when Arya asks, “Didn’t you vow to serve both my MOTHER’s daughters?” – at the word MOTHER you can see a woman in a long green dress w/ long auburn wavy hair with her head down walking past the background gate, resembling Cat. If it’s unintentional, it’s awesome. If not, more awesome. Catelyn cheking on Brienne keeping her vows.