There is no "design flaw" IMO. Something was interfering with the firing pin or sear. From the description I can think of a couple things........ something interfering with the sear, the overtravel on the trigger set a hair too close or foreign matter in it............ more probable the bolt had something in it. Do you pull the bolt apart and clean it? Pretty easy to pull it apart and check everything over. I had one pre-64 that the bolt/main spring was so full of grease and gunk that it would hold the firing pin back after the safety was released.
I guess this is all mute now you've had a 'smith take care of it.
