  In the past few years, the Chaplygin gas (CG) has been considered as an
appealing candidate for unifying dark matter and dark energy into a single
substance. This picture had to face several problems when trying to fit its
predictions with cosmological observations. We point out another potential
problem of this model, showing that, when the CG is described through a
classical scalar field, the corresponding trajectories are strongly unstable.
This implies that an extreme fine tuning must apply to the initial values of
the field in order to end up today with values allowed by observational data.
