Other mistake: When Sherlock was slapped, he later says he was surprised because the girl who slapped him was a "lefty." Then he rubs his right cheek and says it's leathery from all the slaps he's gotten, while the left is soft. This is backwards - anyone hitting him with their right hand would hit his left cheek.
Character mistake: They are in the interrogation room questioning the fiancé of the scientist Natasha when the captain says, "the blood we found at the scene of your fiance's murder, it didn't belong to Bobby Cordero." The characters name was Benny Cordero, which the captain says correctly a few lines later. (00:33:25)
Continuity mistake: When Holmes and Watson are arguing in the kitchen, Watson puts her mug and phone on the counter. A few times during the scene, the mug switches which side of the phone it's on. The first time it's plausible Watson moves it when off camera, but subsequent to that, we can see enough of Watson to tell she doesn't make enough movements to change the position of the mug.
Continuity mistake: Sherlock and Watson are riding in a limousine. They are seated on the back-facing seats of the limo. Thee are 2 passengers seating on the back seats. The street background visible through the back and side windows shows traffic and the limo moving forward. Some shots of Sherlock show the background traffic moving in the right direction (backwards) and one shot shows traffic moving in the opposite direction, as if Sherlock was seating on the back seat or the limo was moving In reverse.
Continuity mistake: When Joan is sitting on the couch folding ties while Shinwell tries on a shirt, she has a folded up blue tie with white dots. When the camera cuts back to her and she's now folding a gray tie, you can see the blue tie with white dots, unfolded, lying on the clothes. Previously there was no tie there. And in the next shot of her, she once again folding the blue tie and the gray tie can't be seen.
Continuity mistake: When Marcus and Joan are watching Gephardt's video, he says he's taken lives. At this point the play bar is by his fingers. It cuts to Marcus and Joan, and we hear the video continue to play for more than 12 seconds. When it cuts back to the video, the play bar is nearly in the same position, when it should be well past his hands and near his elbow.
Continuity mistake: When Shinwell and Sherlock are playing chess, the captured pieces move around in various shots. For example, Shinwell takes Sherlock's knight, he places it closer to him than the pawn. In the next shot the pawn is closer to him. It changes a few more times as do the captured white pieces.
Continuity mistake: The note Watson picks up changes when it switches to a close up view of the writing. For example, you can see one line ends with "you when" and another line ends with "not" (being underlined). In the close up the corresponding lines end with "you when I" and "not to" (not still being underlined).
Continuity mistake: The brownstone has two front doors, an outer door and an inner door. In scenes before this one and after this one, the outer door is made of wood and the inner door has a large glass panel, at the top, but in this episode the outer door has a large glass panel, at the top, so they can see who it is before opening the door.