From: ep. 2.1 (and also brought up frequently in the rest of Seasons 2 and 3)
Event (costume): Sherlock, in an attempt to hide his face from paparazzi, grabs a random hat from a costume rack as he leaves a building. It’s a deerstalker. He mutters, “I’m a private detective; the last thing I need is a public image.” The resulting pictures of him in the hat become iconic and famous.
Reference: Though in the canon, Holmes only wears hats like this when in the
country, as is normal for a Victorian gentleman (and the hat is only mentioned in Doyle’s words once, as an “ear-flapped travelling cap”), the most famous image of Sherlock Holmes in global culture is that of his profile in the deerstalker hat (and meerschaum pipe, which is also not from canon). There’s a glorious line in ep. 2.3, when John is admonishing Sherlock about not being careful enough with his fame. He says, “That’s not a deerstalker anymore; it’s a Sherlock Holmes hat.” And he’s right.