  We discuss an instability in a large class of models where dark energy is
coupled to matter. In these models the mass of the scalar field is much larger
than the expansion rate of the Universe. We find models in which this
instability is absent, and show that these models generically predict an
apparent equation of state for dark energy smaller than -1, i.e.,
superacceleration. These models have no acausal behavior or ghosts.
