As laid out by Simon, in the context of the Neil Sean debate: all hairdressers have two first names. Mark immediately exclaimed, "that's not right, I used to know a hairdresser called David Blair, you can't be called Blair David - oh actually you could, Blair David, you could - I'm sorry, I stand corrected. Well done, that's a good rule."
The rule was then forgotten about, until some three years later, when Simon said it again, and Mark followed it up with exactly the same response.
Seriously. It was spooky.