Several people mentioned how the Arya scene in Episode 7 seemed somehow weird or odd, and I felt the same. I was always thinking about Jaqen being involved somehow (wearing the face of Arya either in this episode or in the next one), but in the back of my mind there was something working.
I tried to remember where I had seen the way Arya is moving before, walking with her hands behind her back... and this is what I was looking for:
I won't even start talking about the clothes both of them are wearing... well, maybe after all it's just a Braavosi custom ;-) .
EDIT: I wasn't aware that there were so many "Syrio Forel is alive / is Jaqen" theories a long time before... so maybe D & D are just playing with us and it's a red herring.
Or it is the real thing... Jaqen was introduced in Season 2 as someone having been arrested and put in the dungeons of King's Landing; something that surely would have happened to Syrio Forel IF he wasn't killed by Meryn Trant.
EDIT 2: I just read Syrio claims to be of the free city of Lorath. The only other person we know to be coming from Lorath is Shae, who by the way is rumoured to be seen in Meereen this season. For a long time I have suspected a Faceless Man being sent after Tyrion by his beloving sister Cersei... no, let's stop here, because this road leads to madness ;-) .