I just lost a post for Current Topics after I spell checked and then attempted to preview. I had forgotten to put in a title, so I clicked on 'back' to put the title in place, but the screen came up with everything in the message space deleted.
I just tried to simulate what you've explained, and I came up with three different results. If there were no misspelled words, not having entered a subject didn't seem to affect the post after trying to preview it and being told I'd forgotten to enter the subject. But if there were misspelled words, sure enough, after trying to preview a post without a subject, the text of the post completely disappeared. I have no idea why, though.
But what's even stranger is that when I did it a second time to confirm that's what will happen every time under those circumstances, I didn't lose the text. :-/
Now, I've tried to preview posts where I've forgotten to enter a subject, and I've never lost the text - but until today I've never spell checked them first. (But then, my experimenting didn't offer solid proof that something having to do with spell checking first had caused the problem.)
I suppose the only thing I can suggest is that for the time being always check to make sure you've entered a subject before you spell check just to be on the safe side. And when I get a chance, I'll see if it's not to complicated to rewrite the code to automatically display "(no subject)" in the subject field when you're creating a new topic just like the IM script does when you're writing someone a new message. At least then, if someone forgets to enter their own subject and it gets posted that way, they can always modify the post to change the subject.