Sherlock Season 4: 20 Easter Eggs & References In 'The Final Problem'
3. The Dancing Men
In the episode's final moments, there is one last blink-and-you'll-miss-it reference to the stories of Arthur Conan Doyle.
As Mary gives her speech, we see John and Sherlock attending to a bevy of cases. One sees them studying a blackboard upon which some stick men in various positions have been drawn.
These stick figures are lifted from The Adventure of the Dancing Men, in which they form a code that Holmes must deduce the meaning of. That same code is replicated here. If you're interested, it reads: "Am here Abe Slaney."