When you preview a new thread without a subject, you get this:
"Server Error
The requested URL cannot be accessed due to a system error on this server.
AOLserver/4.5.0 on http://www.carnageblender.com/"
It's a minor bug, sure, but having an error message is better than not. Ideally, you'd have a javascript one so the user doesn't navigate away from the page and another built in to the next page for those with javascript disabled.