1) Sherlock Holmes was originally going to be called Sherrinford.
The name was altered to Sherlock, possibly because of a cricketer who bore the name. Sir Arthur Conan Doyle, creator of Holmes, was a fan of cricket and the name 'Sherlock' appears to have stuck in his memory. Doyle was also a keen cricketer himself, and between 1899 and 1907 he played ten first-class matches for the Marylebone Cricket Club -- quite fitting, since Baker Street is situated in the Marylebone district of London.
2) The first Sherlock Holmes novel was something of a flop.
The detective made his debut in the novel A Study in Scarlet (1887), written by a 27-year-old Doyle in just three weeks. Famously, Doyle was inspired by a real-life lecturer of his at the University of Edinburgh, Dr. Joseph Bell, who could diagnose patients simply by looking at them when they walked into his surgery. The other important influence was Edgar Allan Poe's fictional detective, C. Auguste Dupin. Doyle wrote the book while he was running a struggling doctor's surgery down in Portsmouth. The novel was rejected by many publishers and eventually published in Beeton's Christmas Annual (named after the husband of Mrs. Beeton). It didn't sell well, and more or less sank without trace.
3) The second Sherlock Holmes novel was the result of a dinner party with Oscar Wilde.
One person who had admired the first novel was the editor Joseph Stoddart, who edited Lippincott's Monthly Magazine. He convinced Doyle, at a dinner party in 1889, to write a second novel featuring the detective, for serialization in the magazine. Wilde, who was another one of the diners, agreed to write a novel for the magazine -- his only novel, The Picture of Dorian Gray.
4) Sherlock Holmes didn't wear a deerstalker hat. Much.
The famous image of Holmes wearing a deerstalker hat is a product of the celebrated images which accompanied the short stories, which appeared in the Strand magazine from 1891. It is when the stories began to appear that Sherlock Holmes became a worldwide sensation. Sidney Paget, who drew the illustrations, had Holmes wearing a deerstalker when the detective went into the country to investigate mysteries at country houses and in small rural villages, but most people think of the detective as always donning the hat when off to investigate a case.
5) Sherlock Holmes is the most-filmed fictional character.
According to IMDb, Holmes has appeared in 226 films and been played by dozens of different actors since the advent of cinema in the late 19th century.
6) Sherlock Holmes is not the most-filmed fictional character.
That is, not if you include non-humans (or partial humans). Dracula has been filmed more times than the great sleuth, at 239 times, but since Dracula is part-man, part-vampire, Holmes is the most-filmed fully human character.
7) Sherlock Holmes doesn't make deductions.
Instead, and if we want to be technically accurate, he normally uses a logical process known as abduction. The difference between deductive and abductive reasoning is that the latter is based more on inference from observation, where the conclusion drawn may not always necessarily be true. However, in deduction, the conclusion drawn from the available data is always necessarily true. But then again, since Holmes's reasoning always seems to be correct, perhaps it is deduction after all!
8) Holmes never says 'Elementary, my dear Watson.'
Not in the canon of original Conan Doyle novels and stories. Holmes says 'Elementary!' and 'my dear Watson' at various points, but the idea of putting them together was a later meme, which possibly arose because it neatly conveys Holmes' effortless superiority to his 'dear' friend and foil. The first recorded use of this exact phrase is actually in a P. G. Wodehouse novel of 1915, Psmith, Journalist.
9) The Sherlock Holmes Museum both is and isn't at 221B Baker Street.
Although the museum in London bears the official address '221B' in line with the celebrated address from the stories, the museum's building lies between 237 and 241 Baker Street, making it physically -- if not officially -- at number 239.
10) There's more to Sir Arthur Conan Doyle than Sherlock Holmes.
Among other achievements, his legal campaigning led to the establishment of the Court of Criminal Appeal. He was knighted for his journalistic work during the Second Boer War, not for his achievements in fiction, law, or medicine. A story he wrote in the 1880s popularized the myth of the Mary Celeste. He wrote historical novels which he prized more highly than his detective fiction. Winston Churchill agreed, and was a devoted fan of the historical novels. Doyle also wrote science fiction romances, such as The Lost World (1912), which would inspire Michael Crichton's Jurassic Park, and, subsequently, Steven Spielberg's film (the sequel to the novel and film being named, in homage to Doyle, The Lost World). Doyle also took up legal causes himself: read Julian Barnes's novel Arthur and George for his most famous real-life case.