A numerical study of the flow in overexpanded planar nozzles shows the existence of Mach reflection hysteresis inside an ideal nozzle while in tapered (constant angle) nozzles it does not appear. When including the geometry of the nozzle in the simulation it becomes evident that flow separation will occur before the transition from regular to Mach reflection for all relevant Mach numbers. The simulation reveals complex changes in the flow structure as the pressure ratio between the ambient and the stagnation is increased and decreased. Detailed examination of the pressure in the region of flow separation reveals an interaction between the shock cell structure and the location of the separation point. The pressure along the nozzle wall downstream of the separation point was found to be less than the ambient pressure with the affect being more pronounced in the case of the ideal nozzle. The formation of closed circulation bubbles may generate reverse flow separation.