The streets of London are regularly covered with tarmac even though this was not invented before 1901. This episode is supposed to take place circa 20 years earlier.
The Gold Sovereign that Irene Adler gave to Sherlock Holmes after the wedding was a type that wasn't minted until 1895.
Specifically, it was a Queen Victoria Veiled Head Sovereign.
Inasmuch as the action in this takes place (depending upon the reference consulted) in the mid to late 1880's, the use of this specific type of coin is anachronistic.