While bluffing to get information from Mrs. Smith about the Aurora, Holmes says "Judge Jeffereys and Dr. Crippen have beaten us to it". This is clearly intended as a joke, as well as perhaps a nod to Holmes' encyclopedic knowledge of crime. Of the two, the only one Holmes would have known about is Jeffereys, a seventeenth century judge famous for handing down executions. Dr. Crippen was an alleged murderer who was accused of poisoning his wife, caught, tried, and executed. However, this event did not happen until 1910. "The Sign of the Four" takes place in 1890, a good 20 years before Crippen's name would have hit the papers.