Although it is not an OpenModelica related problem, I would like to share the following experience.
While checking my own media libraries compatible with Modelica.Media, I have found what I think is a bug in the Modelica.Media.R134a package. The calculation fail is shown in the attached file. Coming from a density between bubble and dew densities, it fails in the calculation of the vapor quality. The origin of the problem seems a bad implementation of the setState_dTX function: the calculation of Helmholtz derivatives and specific enthalpy is done before to check the phase fractions, as if the given density would be a one phase density. This produces a complete fail for calculations in the two phase region coming from density and temperature.
I do not know if this is a known bug, or I am wrong in some point.