2 seasons worth of Arya studying the art of assassination, learning from the best, fighting for the power of magical masks, and her teacher can't handle a simple hit. It was pathetic, she totally botched what should've been a routine kill.

The waif had a clear look at Arya, could've snuck up on her instead of engaging her in chit chat, and then didn't even know how to use a knife to kill her. Plus she couldn't wait around on the bridge for the body to float up? I know Arya has plot armor but come on.