Gorgiano's name is pronounced throughout the episode with a soft G, as though it were "Giorgiano." "Gorgiano" should begin with a hard G.
Enrico Formani is using a modern electric red spotlight in the rafters of the opera house, when everything else is lit by candlelight.
When Watson makes his deductions about the pieces of paper on which requests were printed by the mysterious tenant, he says "The paper was torn off at the side before the printing was done, you can see where the S of 'SOAP' is partly gone." He clearly meant to say it the other way round, that "the printing was done before the paper was torn off", otherwise his deduction makes no sense.