Very very minor detail but it is a testament to the attention to detail of the entire crew of Sherlock, even down to the sartorial choices.

John Watson has a military background and the wardrobe department seems to have been able to incorporate that fact into his civilian clothes. The leather patch on that jacket that he frequently wears is a shoulder guard/pad, on which one rests the butt of a rifle. It is meant to buffer the shock a rifle. Its use now is more for aesthetic reasons, but decades ago, it actually served a real purpose. 

As if I couldn’t be more impressed by this show.