Perhaps Arya was playing the Waif the whole time?
The Waif is supposed to be a badass, and has been kicking Arya's ass from the start... but she fails to kill Arya when she has the chance; She fails to confirm Arya's death when she falls into the river; She fails to trail a bumbling, weaving Arya as she heads through the crowd; she fails to find Arya's hiding spot where she keeps Needle; she fails to use her skills to track and kill Arya properly, instead running through Bravos like a mad person; she even failed at killing Lady Crane properly, as she made too much noise; finally, she fails when she allows herself to be lead into what is clearly a trap... Arya was leaving blood trails on purpose, and the Waif doesn't get suspicious?
It's not an excuse for Arya's resistance to stab wounds (which I think is still ridiculous to complain about, considering the entire nature of the show), but it does seem as if she was toying with/setting up the Waif, even in her injured state.
She knew she was going to lead the Waif to her hiding spot at some point, and she knew she was going to kill the Waif in the dark. Looking at all the evidence, I thought it was a pretty good setup and resolution for Arya's time in Bravvos, even with the unrealistic bodily response to being stabbed.