The Saffman-Taylor instability occurs when a less viscous Stokes fluid is displacing a more viscous one, in a rectangular Hele-Shaw cell. This could be an useful model for the study of secondary oil recovery from a porous medium with low-pressure reserves. In some previous papers was considered a large number of liquid layers with constant viscosities inserted between the initial fluids, in order to minimize this instability. We highlight some strong contradictions related to the linear stability of this flow pattern.