I think Arya knew she would get shanked and wanted to make a show for the waif-that she would die, badly. Having seen the play she knew stage blood would not get the desired effect. Using that knowledge she stuffed her attire with the famous westerosi hamster. The waif thinks she's managed to slip a knife into guts but little did she know it was all hamster guts. The real twist is that Arya is not actually a girl at all (never has been) and thus has no name already-she's simply been a pile of hamsters wearing a girls face. The show foreshadowed this nicely-you'll notice in seasons 1-3, and 5 that no one speaks directly to Arya. They cannot see her, they see only hamsters. The final test of the faceless men is spotting piles of hamsters disguised as humans-a trick employed by nobles to avoid assassinations. The waif has failed and she will die.
GRRM telegraphed this in ACOK, as Syrio says to Ned "why would I try to teach a mound of hamsters stuffed in clothes like some crooked scarecrow how to fight swords good?" Furthermore, the Stark Sigil is actually a hamster with concentric diamonds signifying a big group of hamsters. If R+L = Hamster then Jon may have trouble with legitimizing his rule in season 7 if he's half hamster.