The episode begins with an exchange between Watson and Holmes regarding Holmes' drug use, asking "Morphine or cocaine?" Holmes specifies his preference for a 7% solution of cocaine, and that he requires stimulants to deal with stagnation. This dialogue is actually from the story "The Sign of Four". In Conan Doyle's "A Scandal in Bohemia," Watson merely makes a passing reference to cocaine, curiously referring to "the drowsiness of the drug."
After speaking with the King of Bohemia, Holmes says the great Russian composer Tchaikovsky is "conducting his own works at St James's Hall on Friday." Tchaikovsky conducted in London on four occasions between 1888 and 1893. Given that Conan Doyle published A Scandal in Bohemia in June 1891, we can assume that this episode is set in between 30 March and 11 April 1889 when Tchaikovsky conducted a concert at St James's Hall. The only Friday he would've conducted his orchestra will have been 5 April 1889.
Seen at the end of the episode The Blue Carbuncle when Holmes opens his desk drawer to place the titular gemstone in it, there is a framed photograph of Irene Adler from A Scandal in Bohemia.
Two scenes mimic Paget's original illustrations - the first and last shots of Holmes.
Much of the beginning dialogue is taken from the story "Sign of Four".