In Greek the word ALETHEA means truth.
In Possibility Two (2013), Sherlock mispronounces the name of the bee "osmia avosetta" as "omnia avoseta." In the season finale, he pronounces "osmia" correctly.
The real-world dispute between Greece and the Republic of Macedonia is mentioned in this 2012, with the imagined solution being to call it New Macedonia. It was resolved in a similar way in 2019, but the name was North Macedonia.
When Watson changes the dressing on Sherlock's (Johnny Lee Miller) wound, you can see a 26.2 tattoo on his back below the bullet wound. This is a real tattoo. 26.2 is a reference to the distance of a marathon being 26.2 miles. Johnny Lee Miller is an avid marathoner.