AraiiYama heres a copy pasted response to that
One problem with fanfiction is sometimes writers don't do their research even when it is very simple. A common occurrence is when an author from a country other than the original work's country of origin writes characters as if they are from their own country. An example is Harry Potter where non-British writers (particularly American) make characters act and think like an American would; this also happens vice versa as seen in Fifty Shades of Grey.
Eventually, this accretion of fan-born details and mutations turns into things that "everybody knows" about the series. Those new to or unfamiliar with the original material are frequently confused into believing that it obviously must be canon if so many people mention it, even "facts" of the Epileptic Trees variety. This is especially the case with series that have long runs and which gloss over details which are unimportant to the plot but are of interest to the fans and the fan writers.
One famous example of this is the manga/anime Ranma Β½, released well before the Internet became ubiquitous and when many fans had no easy access to the original source material. All manner of details (including the explanation of Akane's mallet as either a ki attack or as residing in a hyperdimensional pocket, her Flanderization into a "psychobitch", her lethal cooking (rather than being just bad), and the names and fates of the many missing mothers were never touched on in the show but became standardized in Ranma fan fiction over the course of approximately a decade. The process was accelerated and exacerbated by the appearance of fanfiction written by people who had never actually seen the show itself and whose only exposure to Ranma was other fanfiction.
Another famous example is the Harry Potter fanfic The Draco Trilogy, which was apparently so widely read that details such as Blaise Zabini being female and Ginny's name being Virginia were taken to be canon, although they were both refuted by later books.
It's not surprising that fans of some shows occasionally pen FAQs solely to reduce the accumulation of fanon in this way.