  Pinning of superfluid vortices to the nuclear lattice of the inner crust of a
neutron star supports a velocity difference between the superfluid and the
solid as the star spins down. Under the Magnus force that arises on the vortex
lattice, vortices undergo {\em vortex creep} through thermal activation or
quantum tunneling. We examine the hydrodynamic stability of this situation.
Vortex creep introduces two low-frequency modes, one of which is unstable above
a critical wavenumber for any non-zero flow velocity of the superfluid with
respect to the solid. For typical pinning parameters of the inner crust, the
superfluid flow is unstable over length scales $\lap 10$ m and over timescales
as fast as months. The vortex lattice could degenerate into a tangle, and the
superfluid flow could become turbulent. Unexpectedly large dissipation would
suppress this instability.
