Bug preventing posting in my forum

I have a forum the usually works, but from time to time, the user replies to a message, enters a post and gets the error:


when they hit the back button, all their content has been removed.

Has anybody else seen this and is there a fix?  (It's happened to me and I know for sure there are headings and/or message bodies...)

Now that I'm thinking aloud... is there something in the body that might make it invalid?