In Doyle’s “The Engineer’s Thumb,” Sherlock and Watson consult Victor Hatherley, a hydraulic engineer, who recently lost his thumb from an attempt on his life. According to Hatherley, a man named Lysander Stark recruits Hatherly to examine a hydraulic press for removing fuller earth. When Hatherley realizes the machine is not for removing earth and confronts Stark about this, Stark tries to kill Hatherley and cuts his thumb off as he flees. Apparently, Stark headed a criminal organization that illegally minted counterfeit coins with his machine.
Even though Hatherly didn’t know about any criminal activity, there were plenty of warnings signs that accepting this job was criminal. When Stark first approaches Hatherly, the most important aspect
