Revealing mistake: It's a valiant effort, but the scenes of Sherlock & Irene in a London street are shot in New York. Not only is the architecture wrong but you can see a U.S. pedestrian crossing signal over Irene's shoulder. (00:16:15)
Factual error: The number plate on the black cab is GW22 ARY. This won't happen until the year 2022. (00:28:20)
Continuity mistake: When Sherlock first accuses Irene of working for Moriarty, he pins her to a bookcase. When she shoves him away and breaks free, her hair ends up arranged with some strands in front of her. During the following minute, those strands repeatedly switch back and forth from behind her shoulders to in front of them. (00:38:25)
Chosen answer: M broke into the brownstone and left a note. Holmes deduced he was staying at a high-class hotel from the soap scent on the note, and gave a picture from a hidden camera to his "irregulars" on the street to stake out hotels. One of them saw him, so Holmes followed him to his next intended victim and knocked him unconscious with a baton.
Sierra1 ★