When someone who has forgotten to put a subject in, hits submit new topic you give them an error message, wipe out all they have written and return them to an input screen with nothing and no way they can return to any words they have input.
I personally dont care where you would like this posted.
You know about it and setting user's expectations was something I left behind at Oracle in the early 1990's because you now supposed to be aware of caching by now.
For anyone reading it do your work away from the site and then post.
Dont rely on entering the words directly into the site.