Factual error: When Watson is looking at the spiderweb of clues in Holmes' apartment, he is shown a newspaper article about the bombing and doctor's death. The lower left of the clipping shows a story with title "Soviet Government Money For The General Strike." The movie is set in 1891, and the term Soviet first came into use in 1905 to describe a worker's movement in Russia, but a Soviet Government didn't exist until 1917. (00:16:00)
Factual error: When Watson is pinned down by Moran at the arms factory in Germany, he hides behind a large artillery piece. He looks around and sees a pair of hearing protection. They appear to be modern hearing protection ear muffs, wrapped with cloth to make them look period. They have plastic clips to detach the ear cups from the metal band, which gives them away as being modern. They appear to be Peltor style hearing protection.
Chosen answer: That's the key code for his fortune concealed in his library. The one Mrs Watson uses to relay to Inspector Lestrade. Like the book reference numbers in a public library.