Personally I just think people are let down with Arya's portrayal. We've seen Arya be smart, be clever and crafty way before she even knew what the faceless men were. I remember in season 2 how well she hid her gender and identity after Yoren instructed her to hide it after leaving King's Landing. Only two people found out her gender and none found out her name.
And now we go to season 6. She is older, more seasoned and has been learning an assortment of skills from an order of assassins near mythos in their regard.
Just about everytime we've seen her we've seen Arya be smart, vicious, determined, pragmatic, etc.
Then we see her strolling around town completely carefree and then getting stabbed and tossed into a canal.
Wtf. Really? The reason so many people, who are being minimized and criticized as tinfoil hatters, made theories and ideas about what happened episode 7 is because we just could not wrap our minds around Arya's careless behavior in episode 7, her previous cautious behavior in episode 6, and then what happened to her last night in episode 8.
Now some are saying Arya was planning to lure the waif to the cave, but got stabbed first, but if that was the case she would have been much more prepared for anyone speaking or getting close to her, especially when the assassins shes trying to avoid are from a cabal of face changers.
Imagine if when the old woman approached her for the stab, Arya quickly dodges the knife slash and only sustains a slight nick, then disarms the Waif and tussles with her for a second before running and leading her to the cave. Then we see her spring her plan and kill the Waif in the dark.
This Arya would have been the Arya we've known and watched all these years. Instead we get Arya getting caught out, thrown into a river and stumbling into the home of an actress who just happens to be as skilled at suturing wounds as the nurse from Daredevil because she used to get stabby with her boyfriends. Really? It just sounds like such an asspull. She takes all these wounds, and then has a James Bond Casino Royale chase scene with the Waif acting as The Terminator after jumping from a two story building.
Our expectations of Arya being shattered along with this development just left a lot of people(including myself) very disappointed with this episode.