fanlore.org

OGQMa TIL the longest work in the English language is a The Loud House fan fiction with over 16 million words.
gMpwb TIL a bunch of Harry Potter fans built a legitimate cult-like religion called Snapeism, based on the idea that Severus Snape was an immortal deity whose spirit inspired JK Rowling to write the Harry Potter books
d8OV9 TIL that a "Mary Sue" character - usually a writer self inserting themselves into a story where they can do nothing wrong - is named after a Star Trek fanfic where a 15 year old Lieutenant Mary Sue ran the ship, won the Nobel Prize, and her birthday was a national holiday each year.