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 Detective Bell receives a phone call from Gregson in Holmes' house, he rushes upstairs and bangs on the bathroom door - Sherlock is inside, he said he wanted a shower. Bell rattles the handle, but finds the door locked, so he kicks the door in. But Sherlock didn't lock the door. The lock is above the handle and Sherlock did not use it.
Continuity mistake: While Joan and Sherlock are chatting, Joan is trying to make peanut butter and jelly sandwiches, but the shots alternate between the two and Joan repeats her movements, wipes the peanut butter over and over on the bread, and changes her mind with the jelly bread.
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.
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.