If you look at the RecentChanges for OG London, you'll see it's filled with dozens of category and locale autocreations. Because I don't think anyone would be bored enough to do such a thing, I reckon it's a web robot of some kind that's followed lots of links to nonexistent categories and consequently brought them into existence.

I think this exposes a vulnerability in the mechanism; numerous of the new categories were erroneous.[0] I propose that autocreation be made a user choice; namely, something like,

The category you selected does not exist, but we can create it for you if you would like.

[Create Category]

with "Create Category" being a form submit button.

[0] Actually, it was quite useful as it exposed lots of pages that needed to be tidied, but I'm not sure that would rate as a "feature"!