I was recently thinking about fanon, canon, and authorial intent (author's intention that may or may not match what is shown in the material). A thing that really annoys me is the large amounts of fanon being mistaken as canon and even more so the fanon which outright contradicts canon.
I do not hate fanon, in fact I like it quite a bit and have my own pet theories for events not fully described in the source material. I realize that they are not strictly speaking canon but they all at least do not contradict canon. I also acknowledge that they are just my theories and interpretations rather than what is strictly in the source. Good fanon expands upon canon, fills out parts not needed for the original story, and occasionally explains inconsistencies in the source material.
Below are seven things all of which are fanon, the reason that these aren't on my other list is because while fanon they do not entirely contradict the manga.
1. The engagement is to unite the Tendo and Saotome schools of martial arts.
- The reason this is fanon is because it is commonly assumed that the reason for the engagement is to unite the two schools of martial arts but the source material never says that. This particular bit of fanon rather annoys me because I never interpreted it that way. The way I always viewed it was that it was a joining of the families and that both families had a practitioner of martial arts was coincidental. Had it been about joining the schools no marriage is needed, they could just train each other in their respective styles and Akane would be the only viable choice. That is not the case, Ranma was given a choice by Soun at first and later they were okay with the engagement being switched to Nabiki (until Kasumi pointed out that Nabiki plans on selling the dojo, house, and land to have fun). That doesn't mean that martial arts has nothing to do with the engagement though since Soun and Genma's training together probably is what made them decide to unite their families and the Tendos need a male heir to run the dojo (the Japanese are more sexist than many other countries and a dojo run by a woman would not attract many students).
2. Ranma was 15 at the start of the manga.
-This is one of my favorite theories. In Ranma's grade level (first year; USA equivalent to 10th grade) it is not unusual to be 15 (it depends on what day your birthday falls on) and Ranma's age is not given in the manga until the mushrooms of aging arc where he states he would lose a year were he to eat the 17cm mushroom rather than a few months implying that he only recently turned 16.
3. Nabiki's characterization.
-While many of the characters in fan fiction are fairly different from how they are portrayed to be in the manga, Nabiki's characterization in particular rarely resembles anything like how she is portrayed to be in the manga. She is almost universally made to be a much nicer and less self centered person in fan fiction even though the way Nabiki is in the manga is how some teenage girls in Japan really are (what with enjo kōsai and such). One example, of the many possible, of how her fan fiction characterization differs from her manga incarnation is in fan fiction she will keep her word particularly in business deals. That bit of fanon is straight out contradicted by the manga for example Principal Kuno hires Nabiki for one of his schemes she does not keep her word instead she takes his money and then runs.
4. Characters having knowledge of things that they should have no way of knowing at all.
- This is not a case of fanon so much as an error in keeping track of who knows what. The reason I'm listing it as fanon is because I occasionally see the same errors made in multiple fics. For example I recently read a fic in which Kasumi had first hand knowledge of what happened during the battle with Saffron even though she was a few thousand miles away at the time or Nabiki having knowledge about the Senkens, particularly the Yamasenken, even though she never even met Ryu Kumon and didn't see the battle (only Akane, Genma, and Soun did). Nabiki in particular has this error apply to her in fan fiction sometimes explained by her having minions (which she does not in the manga - though it can be argued that some characters do for example Kodachi in the form of her Gymnastics team who she even had help her in battle) and manga Nabiki does not search for things to blackmail people (Ranma is much more likely to do that since he has searched multiple peoples places) and is often out of the loop (she did not know Happosai weakened Ranma until she looked over Tatewaki's shoulder and read it).
5. Shampoo's people, the Joketsuzoku, are matriarchal and treat males as second class citizens.
- This bit of fanon probably comes from the fact that VIZ calls her people Amazons and that term is associated with certain things. Pretty much everything about how her people are governed and most of their culture is fanon since almost nothing is shown or told to us in the manga. Though Cologne is not called matriarch ever and a society that was extremely matriarchal would not allow Mousse to be as strong as he is or get away with doing what he does.
6. Ukyo hires other people to work at her restaurant when she isn't there.
- Again a bit of fanon I like in that it explains how she can go to school, take trips (like the one with Ryoga to a spring), and go on trips with Ranma and still keep her shop open.
7. Ranma considers Nabiki's clothing to be better than Akane's.
- I've only saw this in one fic but it was strange enough that it caused me to pause and is probably the thing I remember best about the fic. I can see Ranma saying so to purposely anger Akane but other than that I find it very unlikely since Akane cares a lot more about clothing than Nabiki is shown to and half the stuff Nabiki wears is shown to be Akane's.