  We study cosmological perturbations in a model of unified dark matter and
dark energy with a sharp transition in the late-time universe. The dark sector
is described by a dark fluid which evolves from an early stage at redshifts $z
> z_C$ when it behaves as cold dark matter (CDM) to a late time dark energy
(DE) phase ($z < z_C$) when the equation of state parameter is $w = -1 +
\epsilon$, with a constant $\epsilon$ which must be in the range $0 < \epsilon
< 2/3$. We show that fluctuations in the dark energy phase suffer from an
exponential instability, the mode functions growing both as a function of
comoving momentum $k$ and of conformal time $\eta$. In order that this
exponential instability does not lead to distortions of the energy density
power spectrum on scales for which we have good observational results, the
redshift $z_C$ of transition between the two phases is constrained to be so
close to zero that the model is unable to explain the supernova data.
