This one comes up pretty much any time someone finds out that I spent a lot of time studying the Medieval period when I was getting my degree. The conversation usually goes something along the lines of, "Oh, you studied Medieval church politics. Did they cover Pope Joan?" *smirk* *wink wink* You know, because we only studied *Catholic Church Approved* history at my... secular... state... university.
For those of you who haven't heard of this one (it seems to be favored in the "I watch Game of Thrones so now I'm a Medievalist crowd" {no offense to the people who like Game of Thrones, but it's not exactly a shining example of Middle Ages accuracy}), Pope Joan is supposed to be a woman who managed to hide her sex and reign as Pope for a period of a little over 2 years. But she got pregnant and delivered a baby in the middle of a parade of some sort and then was stoned to death by the crowd. Or something like that, there are a few variations of the tale.
The story often goes on to elaborate that the entire world has covered up her existence conspiracy-style because it would be embarrassing to the Catholic Church to admit her existence, so it's a secret known by a select few and the person I'm talking to just happen to be one of the few in the know.
*facepalm*