In the Game of Thrones Season 5 Finale Arya Stark becomes blind, because she killed a man who was not hers to kill. But I am not convinced that that is exactly the reason for her blindness. Can anyone tell the exact reason why Arya lost her vision?
The man with Jaqen's face actually explained this one pretty clearly (as seen in the transcript):
The faces are for no one. You are still someone. And to someone, the faces are as good as poison.
It's a side effect of wearing a face before being ready for it, before having completed her training and become "no-one".
(note this is pretty different to the books, where it's a seemingly planned part of her training)
But why did the face have no effect until that exact moment? It would have been hours ago that she wore it, with no hint of ill effects.
Erm, yeah, no answer to that one. There have been quite a few contrived coincidences this series, and this isn't even the first moment where something waits until the exact right dramatic moment to kick in with a character somehow predicting when this would be down to the exact second, as if they'd been counting seconds for the several hours that had passed since the initial cause (e.g. the sand snakes' poison and Bronn - '14,398 seconds since I cut him, 14,399, 14,400, now! "How is your head?"'). So I suspect there's no reason beyond "the producers thought it would be dramatic".
Maybe the face has side effects which are triggered by certain emotions like grief or regret, and a "no-one" is not supposed to have such emotions? In the books, donning a face also brings emotions and memories from the person it was taken from, so it makes sense that avoiding severe psychological or sensory side effects would be difficult. I rather feel like I'm making excuses for them now though...
I think she was supposed to realize what it was to, sort of, use it correctly. And like someone already said, probably because of the life she wasn't meant to take. (An eye for an eye, maybe?)