  Event-by-event hydrodynamic simulations of AA and pA collisions involve
initial energy densities with large spatial gradients. This is associated with
the presence of large Knudsen numbers ($K_n\approx 1$) at early times, which
may lead one to question the validity of the hydrodynamic approach in these
rapidly evolving, largely inhomogeneous systems. A new procedure to smooth out
the initial energy densities is employed to show that the initial spatial
eccentricities, $\varepsilon_n$, are remarkably robust with respect to
variations in the underlying scale of initial energy density spatial gradients,
$\lambda$. For $\sqrt{s_{NN}}=2.76$ TeV LHC initial conditions generated by the
MCKLN code, $\varepsilon_n$ (across centralities) remains nearly constant if
the fluctuation scale varies by an order of magnitude, i.e., when $\lambda$
varies from 0.1 to 1 fm. Given that the local Knudsen number $K_n\approx
\frac{1}{\lambda}$, the robustness of the initial eccentricities with respect
to changes in the fluctuation scale suggests that the vn's cannot be used to
distinguish between events with large $K_n$ from events where $K_n$ is in the
hydrodynamic regime. We use the 2+1 Lagrangian hydrodynamic code v-USPhydro to
show that this is indeed the case: anisotropic flow coefficients computed
within event-by-event viscous hydrodynamics are only sensitive to long
wavelength scales of order $\frac{1}{\Lambda_{QCD}}\approx 1$ fm and are
incredibly robust with respect to variations in the initial local Knudsen
number. This robustness can be used to justify the somewhat unreasonable
effectiveness of the nearly perfect fluid paradigm in heavy ion collisions.
