- A very popular bit of fanon in the Sherlock Holmes fandom is that Dr. John H. Watson's middle name is Hamish; this theory was first devised by Dorothy L. Sayers in order to explain why Watson's wife calls him James in one story although his first name was previously stated to be John (Hamish is the Scottish form of James).
- It's fairly established fanon that Holmes' parents were called Violet and Sanger and that he at some point was part of a Shakespearean acting troupe that toured America.
- His older brother Mycroft is head of the proto British secret service. This sometimes assumes that the Diogenes Club is actually a front for some kind of spy organisation, an idea explored most deeply in a series of stories by Kim Newman.
- The eldest of the Holmes brothers is called Sherringford (the name Arthur Conan Doyle gave to Sherlock in early drafts), a country squire. Many fans assume, based on minor hints in the stories, that Sherlock comes from the landed gentry. This would imply that as well as Mycroft the civil servant, there must be a third eldest brother looking after the ancestral land.
- More Sherlock Holmes fanon; Watson had three wives, Holmes and Irene Adler met in Montenegro while he was faking his death between The Final Problem and The Empty House and fathered a child who would grow up to be Nero Wolfe, that the King of Bohemia was Edward VII, that Holmes worked on the Jack the Ripper case (and it was Watson who secretly solved it), that Holmes's retirement to bee-keeping was in the hope of creating "royal jelly" (believed then to be a sort of Fountain of Youth) and that Holmes spent the last decade of his life fighting Nazis before dying at the ripe old age of 90. All of this is present in W.S. Baring-Gould's tongue-in-cheek "biography", Sherlock Holmes on Baker Street.
- The idea that Watson had more than one wife comes from several stories. In "The Adventure of the Blanched Soldier", Holmes, for once narrating a story, talks about Watson having "deserted me for a wife". The story is explicitly dated to 1903, well after the death of Mary Watson nee Morstan. "A Scandal in Bohemia" and "The Five Orange Pips" both briefly mention Watson being married before his canonical meeting with Mary. There is much fannish argument about who these other two (at least) might be.
- The Exploits of Sherlock Holmes, written by Arthur Conan Doyle's son, Adrian, and his editor, John Dickson Carr, is in an awkward place where half the fandom considers it fanon, and the other half considers it Fanon Discontinuity.
- For some reason there's a train of thought in parts of the fandom that either Sherlock or Watson (most commonly the former) are Female-to-Male Transsexuals.
- People who join the 'Baker Street Irregulars" have to write an essay. Rex Stout's was titled "Watson Is a Woman".
- Many adaptations talking about Sherlock's family-life (Young Sherlock Holmes, The Seven Percent Solution, Sherlock) seem to be oddly agreed on the fact that a young Sherlock Holmes deduced that his father was having an affair, told his mother and ended up ruining the family.
- For some reason, Holmes/Watson is so widespread, it's often mistaken for actual Canon by fandom newbies. Adaptations don't help. Is considered Serious Business by some fans to the point essays are written about the "subtext". (In a series where "ejaculation" means "sudden exclamation" and nothing else, at that.)
- On the other hand, just about every non-Doyle author to write multiple Holmes stories will eventually buy into the fanon that his relationship with Irene Adler was more than intellectual, and that rather than the courtesan she is described as in "A Scandal In Bohemia" she was a much more modern and active kind of "adventuress".
- Watson's appearance is never described beyond being "brown as a nut and thin as a lath" in A Study in Scarlet, and that after having just returned from severe illness abroad. Later, Watson's describtion was given as " middle-sized, strongly-built, square jaw, thick neck, and mustached." Nowadays, though, it's generally accepted that he was blond - helped along, perhaps, by David Burke of the Granada series and Vitaly Solomin of the Russian series, not to mention Jude Law and Martin Freeman.
- Another piece of Fanon that's spreading is the given name "Geoffrey" for Inspector G. Lestrade. This originated with Marcia Wilson and has been picked up by an unknown number of fans, including Aleine Skyfire and Riandra.
- The Agent - Sir Arthur Conan Doyle; a reference to the Literary Agent Hypothesis.
- The Master - Sherlock Holmes.
- Wait, shouldn't Moriarty be The Master?
- The Good Doctor - Watson. No relation.
- Calling someone (usually sarcastically) solving some mundane problem "Sherlock Holmes" is an Older Than Television meme.
- Exclaiming "No shit, Sherlock" when someone states the obvious.
Common Fanfic TropesExamples should be tropes that show up a lot in fanfic which are not just taken purely from canon:
- Sherlock (BBC series)
Granada TV Series
Guy Ritchie film series
- Memetic Mutation
- "You've never complained about my methods before." "I've never complained! When have I ever complained about you...practicing the violin at three in the morning, or your mess, your general lack of hygiene, your experiments on my dog, or the fact that you steal my clothes?"
- "Get that out of my face." "It's not in your face, it's in my hand." "Get what's in your hand out of my face."
- Be...a lady...* oof!*
- "Don't get excited."
- YOU wear a jacket.
Elementary TV Series