Thermodynamics of the unified dark fluid with fast transition

In the so-called unified dark fluid models, the dark sector gets simplified because dark matter and dark energy are replaced by a single fluid that behaves as the former at early times and as the latter at late times. In this short paper we analyze this class of models from the thermodynamic viewpoint. While the second law of thermodynamics is satisfied, the first two derivatives of the entropies of the apparent horizon and of the energy components suffer such a sharp oscillation that doubts are raised about the soundness of this class of models.