Arya stole a face from the House of Black and White, and used it to carry out the murder of a man that the Many-Faced God had not marked for death. There were obviously going to be repercussions. Only life can pay for death, and it is briefly teased that Arya is going to pay with hers. Instead it is the man bearing the face of Jaqen Hghar who dies, although he reappears again on a different body. Arya tears faces off the old Jaqen until her own appears, at which point she goes blind. This punishment mirrors her murder of Trant, as his eyes were the first things she stabbed. Whether it is a permanent development in the show remains to be, er, seen, and while it is better than death its still a hefty price to pay for the murder of Meryn Trant.