What does this mean?

You'll be delighted to know that there are no spoilers in this post. Game of Thrones is in full flight now and episode four was the highest viewed episode of the show ever, grabbing the attention of over 10.17 million people. People are watching Game of Thrones extremely close making sure that they don't miss out on any small details that may occur. However, we can guess that a lot of you missed out on this vital moment in the show last weekend but this Reddit user did not. With a username of GrennsGal, they noticed something odd at the moment where Arya tells Brienne:

"You swore to serve both my mother's daughters.”

At that exact moment, a shadow walks by in the background and fans believe that the ghost is that of Catelyn Stark who died in season three's famous Red Wedding. Don't believe us? Have a look for yourself around 00:12-00:15... https://www.youtube.com/watch?time_continue=17&v=9bI6h_9K4s0 Can't see it? How about now? GrennsGal explains their theory:

If you watch the sparring scene in S07E04 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."

If you look closely at it, it doesn't look like an actual person but more of a hologram of someone that was placed into the video. What do you think? Is there something more to this theory or is it just a theory?