Dense gases with complex molecular structure may behave in an unexpectedly manner, in some cases on the contrary to regular fluids. In the present study, supersonic as well as subsonic boundary layer flows of dense gases are numerically solved. It is shown that a supersonic accelerating/decelerating dense gas may be negative/positive for the boundary layer evolution. In the latter case, special attention is given the phenomena where the wall shear stress almost vanishes in a single point but then recovers and increases. Incompressible high Reynolds number flows past a slender airfoil have the distinguishing property that there exists a critical value of the angle of attack where the wall shear stress on the suction side vanishes in a single point but immediately recovers. This phenomenon is now commonly referred to as marginal separation. In this work it is shown that a laminar boundary layer on a flat plate subjected to a linearly retarded external flow may experience a similar behaviour if the molecular complexity of the medium is sufficiently large. The role of the critical angle of attack is played by the critical value of the free stream density on a given isentrop. As in the case of a slender airfoil it is possible to construct an asymptotic theory which allows to calculate small separation bubbles which form if the free stream density is smaller than the critical density.