Sherlock Holmes has appeared many times since Sir Arthur Conan Doyle first started publishing stories about the detective in 1887. Although various aspects of the character have differed with each iteration in literature, TV, film, and beyond, Sherlock's famous address has mostly stayed the same: 221B Baker Street, London. When Doyle first wrote about the sleuth, that location didn't actually exist, because the street numbers on Baker didn't go past 200.
For years, fans would send him letters, some for fun and some genuinely asking for his services as if he were real, but they couldn't be delivered. Then, in the 1930s, London's street numbers underwent some reorganization, resulting in the birth of a real 221B Baker Street, which just so happened to belong to a bank. Interestingly, the institution was a good sport about the fan mail, hiring somebody to serve as Sherlock's "personal secretary." This person was responsible for explaining that he had retired from detective work to go raise bees in the country.
In the 1990s, a Sherlock Holmes museum opened up down the street and tried to assume control of this postal arrangement, which the courts didn't go for, resulting in a custody battle over fan mail sent to a detective who, however famous, has never been real. Eventually, the squabble ended when the bank moved its headquarters and 221B Baker Street was awarded to the museum, who has been informing interested letter writers of the fictional detectives whereabouts ever since.