Factual error: In Paris, Colonel Moran gives a ticket for Don Giovanni to Moriarty. The ticket lists the performance date as "Mardi 7 Novembre." Mardi means Tuesday in French, except in 1891 November 7 was a Saturday. (00:52:20)
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: At one point, Sherlock uses a tube of lipstick to sabotage a gun. The film is set in the late 19th century, and tube lipstick wasn't available until 1915.
Continuity mistake: Initially during the forest chase scene, Watson is on Sherlock's right side and Madam Simza on the left. After Little Hansel is shot which throws them forward, Watson is now on Sherlock's left side and vice versa.
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.
Revealing mistake: During the shootout at the German arms factory, one guard is firing an M1888 Mauser rifle at Holmes, Watson, etc. However, when he fires it, the bolt handle is up, but we still see a muzzle flash from the rifle. This would indicate the muzzle flash to be computer generated and not a blank from the rifle.
Factual error: The map shown in the factory scene uses the post WWI Western boarders of Germany, when it should have included northern Sleswik (today in Denmark) as well as Alsace Lorraine (today in France). (01:18:30)
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.