In a feature on the DVD there is a segment concerning the age of the character "Mina/Cassie" and even when Sherlock asks her about her age in the episode she becomes quite cryptic. In real life, this episode aired exactly 5 days before the actresses 18th birthday.
In this episode a 10 year old girl is abducted and held for a decade. When she escapes she tells the authorities that she was kept in a basement, but was occasionally allowed out in public, with her captor. This is almost exactly the same as the real life events which happened to Natascha Kampusch, in Austria, who was abucted on the 2nd of March 1998, when she was 10. She was held in a cellar, but was occasionally taken out in public by her captor. She managed to escape on the 23rd of August 2006.
Holmes makes elephant poop coffee. Black Ivory Coffee is the market leader in elephant poop coffee and costs over $1100. per kilo. Compare that to the $700. a kilo for the more pedestrian palm civet poop coffee.
The song at the end of the episode is All we do by Oh Wonder.
Interestingly, the song at the end, All We Do, is the theme music of the British show Unforgotten, which is about solving cases from the past.