Jay J. Armes, who played Stoner, was actually missing both hands at age 11 when he and a friend stole some railway torpedoes and they detonated. He turned his life around and became a highly-decorated private investigator and an author, and also created a line of action figures with prosthetics in conjunction with Ideal.
In his autobiography, Jay J. Armes refers to this episode as "my only caper on the wrong side of the law".
The opening scene of the funeral procession with the casket falling out of the hearse was not planned. The writing partners of the show, Glen Olson and Rod Baker, were viewing the filming of the fish-tailing of the hearse. When filming was ready to move to another location, they said something to the effect of "why don't they film the coffin pitching out of the hearse". Executive producer Leonard Freeman was on set and overheard them. He said it was a great idea and wanted the shot. The director complied and was not upset with the meddling.
After McGarrett discovered who had been killing HPD officers and leaving M-1 rifles bearing gold plates stamped with their names, he went to a tenement in search of Kurt Stoner. Before doing so he told Danno to call for backup, but he failed to wait and entered the room by kicking in the door not knowing if Stoner was present. In the real police work, this would have been deemed foolish and possibly a violation of departmental regulations.
When McGarrett is in Stoner's room near the end of the show, the photographs on the wall showing Stoner with hands were provided by Armes himself when he was wearing prosthetic arms and hands for his real-life detective work.