  We discuss the interplay between a slow continuous drift of temperature,
which induces continuous phase separation, and the non-linear diffusion term in
the $\phi^4$-model for phase separation of a binary mixture. This leads to a
bound for the stability of diffusive demixing. It is argued that these findings
are not specific to the $\phi^4$ model, but that they always apply up to slight
modifications of the bound. In practice stable diffusive demixing can only be
achieved when special precautions are taken in experiments on real mixtures.
Therefore, the recent observations on complex dynamical behavior in such
systems should be considered as a new challenge for understanding generic
features of phase-separating systems.
