  We discuss the onset of the non-linear regime in the context of unified dark
matter models involving a generalised Chaplygin gas. We show that the
transition from dark matter-like to dark energy-like behaviour will never be
smooth. In some regions of space the transition will never take place while in
others it may happen sooner or later than naively expected. As a result the
linear theory used in previous studies may break down late in the matter
dominated era even on large cosmological scales. We study the importance of
this effect showing that its magnitude depends on the exact form of the
equation of state in the low density regime. We expect that our results will be
relevant for other unified dark matter scenarios particularly those where the
quartessence candidate is a perfect fluid.
