In this age of off-the-shelf SMPSs with internally compensated feedback loops on large PCBs, one can actually create unanticipated problems with inter-loop oscillations. The PFC/SMPS combination is the most common example, but the PFC stage has a such low bandwidth that it appears to be more of a battery. On a large PCB, it is a little more complicated where higher bandwidth SMPSs are cascaded. Buying off-the-shelf solutions can present some surprises.