My guess is they're swerving us with Arya, but even if that ends up being the case, I think I'm going to end up pretty dissatisfied.  First, they've played Arya's arc straight for multiple seasons now.  We all thought she had something up her sleeve with the Waif, but nope, she was just an idiot, and somehow lucked her wait into winning a duel with an assassin after getting stabbed in the stomach.  Second, they've given us few in-world hints that there's mischief afoot.  We can hypothesize that Arya and Sansa may be working together, and that Bran might have informed them of Littlefinger's treachery, but where is the groundwork?  If you're not working with a bunch of meta information, how could you ever guess Arya's behavior was an act?  All my swerve guess is based on is a bit of genre savvy, and an attempt to justify otherwise ludicrous behavior.