  Using dynamical Monte-Carlo simulations we observe the occurrence of an
unexpected shock wave in driven diffusive systems with two conserved species of
particles. This U-shock is microscopically sharp, but does not satisfy the
usual criteria for the stability of shocks. Exact analysis of the large-scale
hydrodynamic equations of motion reveals the presence of an umbilical point
which we show to be responsible for this phenomenon. We prove that such an
umbilical point is a general feature of multi-species driven diffusive systems
with reflection symmetry of the bulk dynamics. We argue that an U-shock will
occur whenever there are strong repulsive interactions such that the
current-density relation develops a double-well and the umbilical point becomes
isolated.
