The inability of the pilot-in-command (PIC) to recover from an inadvertent spin following a stall demonstration for reasons that could not be determined because aircraft and engine examinations did not reveal any anomalies that would have precluded recovery from the spin. Contributing to the severity of the accident were the PIC’s failure to remove the airframe parachute system safety pin before takeoff, the exceedance of the left-seat weight limitation, and the location of the parachute system activation handle behind the PIC’s seat, which prevented easy access during the uncontrolled descent.