The worst fanon items are those that are made up by the fans, promulgated for years without any support from the show, then rewritten by the show - exactly like the "Spock is the first Vulcan in Starfleet" example of Lance's.
I was not around during the 1970s when Trek was only in reruns, and the fans had only their own ideas about character names and histories and "plot holes" that needed to be filled. I wasn't around when Franz Joseph was releasing blueprints for the Enterprise. I'm sure it was an exciting time, when the three seasons of TOS were the entire Trek universe, and fans were free to play in that universe to their heart's content. But to expect the producers to hold to fanon that developed in that void is unrealistic.
While I agree it was unrealistic to expect that, it sure would have been nice. Would it have killed the producers of Enterprise to find a story adviser who had a good knowledge of the fanon that was developed during those years, and throw us a bone? Manny Coto did it in the last season. While Berman was in charge for the first 3 seasons, a lot of us kept going "What?! But that's not... Aw come oooonn!!"