The do not slip with flaps placard normally indicates that you may not slip with FULL flaps, but does not limit slipping with partial flap extension. The reason is that the full flap extension, up to 40 degree on some 172's, can effectively blank out airflow over the tail in a slip and cause a tail stall, as earlier stated. Check the POH; I believe that the limitation was for full flap only, but this is from the dark corners of my mind, as it has been a while since I have flown a 172.