First of all, let us say that Sherlock Holmes fanon is Serious Business, with the earliest examples dating to the early 1900s. The tradition of examining the stories as if they were really Dr Watson's accounts of true events, and trying to patch up his inconsistencies, anachronisms and evasions, is often referred to as "The Game". Aspects of Holmes' life that have been "deduced" by influential figures like Dorothy L. Sayers, Ronald Knox, William S Baring-Gould and Nicholas Meyer have a tendency to show up in other people's Holmes pastiches. Some aspects have ascended all the way to Word of Dante levels, being repeated without writers realising they're not part of the original Canon at all. (It should be noted that there is a theory that Knox and Sayers, in particular, started this kind of fan speculation initially as a parody of rationalist biblical criticism.)
Related to YMMV, except these particular fan-reactions and interpretations are quite widely entrenched and many have been around for upwards of fifty years and some close to a century.
- 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 Siger and that he at some point was part of a Shakespearean acting troupe that toured America. ("Siger" is a bit of wordplay Baring-Gould invented. Holmes spent part of his "dead" years as an explorer named Sigerson - Baring-Gould retroactively made it an Incredibly Lame Pun that verged on Hiding In Plain Sight.)
- 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 or Sherrinford Holmes (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 (a common job for second sons), there must be a third eldest brother looking after the ancestral land.
- Irene "the Woman" Adler has a whole life outside her one appearance. Just about every non-Doyle author to write multiple Holmes stories will eventually buy into the idea 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". Common examples for this character alone include:
- That the "King of Bohemia" was actually Edward VII, and Watson's protecting his name. (The real King of Bohemia was also the Emperor of Austria, Franz Joseph I, who was nearly sixty at the time of the story's publication.)
- Almost everybody assumes that Watson was also protecting her when he refers to her as "the late Irene Adler" in the story, which he is recounting only three years after the events in 1888. Particularly since this would rule out her and Holmes meeting during his "Great Hiatus" from 1891 to '94.
- Baring-Gould had Holmes and Irene Adler meet in Montenegro while he was faking his death between The Final Problem and The Empty House and father a child who would grow up to be Nero Wolfe (an idea first put forward by John D Clark, and expanded by John Lescroart who wrote two pastiches featuring a young Nero Wolfe under an assumed name that also connects more closely to Holmes.)
- Holmes working on the Jack the Ripper case is a perennial favourite. Common plots include Holmes discovering the truth and being unable to reveal it, Holmes failing to solve it, Watson solving it and at least one case of Holmes being the Ripper himself.
- 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.
- 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 née Morstan. And "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.
- There's a long-running train of thought in parts of the fandom that either Sherlock or Watson (most commonly the former) is a trans man. The earliest example seems to be Rex Stout's essay for membership of the Baker Street Irregulars, titled "Watson Is a Woman".
- Many adaptations talking about Sherlock's family-life (Young Sherlock Holmes, The Seven-Per-Cent Solution, Sherlock) seem to be 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. The earliest (and grimmest, given that it results in a murder-suicide) example seems to be Nicholas Meyer's The Seven-Per-Cent Solution.
- 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.)
- 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 description 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.
- Another perennial favourite is retroactively connecting standalone stories to Holmes' nemesis Professor Moriarty. Almost any story involving a criminal conspiracy has been "revealed" as Moriarty's scheming (or acting as a "consulting criminal") by some writer.
- 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)