Lady Stoneheart is the zombie Catelyn Stark in the books.
While Arya and Brienne sparred — as Sansa watched from above — in Sunday's episode, one observant viewer thought they may have spotted Mama Stark returning to check up on her two daughters."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 [with] long auburn wavy hair with her head down walking past the background gate, resembling Cat," wrote Reddit user GrennsGal. "If it’s unintentional, it’s awesome. If not, more awesome. Catelyn cheking on Brienne keeping her vows."