At the end of the episode, Sherlock says that the bees should not be able to produce offspring because they are different species. That is not correct, different species can produce offspring (e.g. a mule) but the offspring will be unable to reproduce (e.g. mules are sterile).
While Macedonia is indeed a candidate member for the EU, admittance to the EU would not automatically mean Macedonia would get the Euro as a currency. The procedure for switching to the Euro is separate from becoming a EU member.
In the final scene, Sherlock claims to have bred a new species of bee from a single individual. Assuming that individual was a queen, any cross breeding would have produced a hybrid variety or, at best, a subspecies, both of which could easily produce viable offspring and would NOT have been sterile, as he suggested. Creating a new species would require tens or hundreds of thousands of generations.
This is an 'elementary' detail which the encyclopedic Holmes would not have stumbled over.