  We discuss the quantum phase transition from the Mott-insulator state to the
density-wave state for cold Bose atoms in a 2D square lattice as the lattice is
adiabatically tilted along one of its primary axes. It is shown that a small
misalignment of the tilt drastically changes the result of the adiabatic
passage and, instead of the density-wave state, one obtains a disordered state.
Intrinsic relation of the problem to Bloch oscillations of hard-core bosons in
a 2D lattice is illuminated.
