Whenever somebody adds a thread using the sidebar, it should give an error if the person doesn't change the category. Would get a lot less posts in the wrong section while people who browse through the forum index will still get their current category from the main bar (since the URL changes to include that subforum id).

Thoughts. All it would do is change the URL to thread/add/0 or -1 and then once it finds out there's not category with that id, it'll give you an error saying "Please choose a category for your post."