I was trying to think of examples of 'fanon' that were so all encompassing that people accepted them as fact even though they had never been stated as such on screen.
One which occured to me was the old chestnut about Spock being the first Vulcan in Starfleet. Somebody came up with this one back in the 1970s and it was accepted wholeheartedly by a lot of fans for decades... right up to the point where Enterprise
"contradicted" it, and people were in an uproar. Despite, you know, it never having actually been said on screen in the first place.