Before the seventh season of Game of Thrones premiered, author George R.R. Martin was asked about his least favorite book-to-show changes. “I think one of the biggest ones would probably be when they made the decision not to bring Catelyn Stark back as Lady Stoneheart,” he said. “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.”
The delayed arrival of Lady Stoneheart (think: Zombie Catelyn Stark) has become an in-joke among Game of Thrones viewers, but it’s extremely unlikely she’ll appear on the show. There was a point last season where it might have made sense, but no dice (instead, we got Lem Lemoncloak giving a familiar speech). That hasn’t stopped some fans from searching for Catelyn, though.
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. (Via)
Here’s the scene in question.