  The AdS/hydrodynamics correspondence provides a 1-1 map between large
wavelength features of AdS black branes and conformal fluid flows. In this note
we consider boundaries between nonrelativistic flows, applying the usual
boundary conditions for viscous fluids. We find that a naive application of the
correspondence to these boundaries yields a surface layer in the gravity theory
whose stress tensor is not equal to that given by the Israel matching
conditions. In particular, while neither stress tensor satisfies the null
energy condition and both have nonvanishing momentum, only Israel's tensor has
stress. The disagreement arises entirely from corrections to the metric due to
multiple derivatives of the flow velocity, which violate Israel's finiteness
assumption in the thin wall limit.
