Revealing mistake: When the movie starts, there's an aerial view of London's rooftops. The smoke coming out of the very first chimney doesn't move, revealing it's a painting.
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.
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.