have run into this lots in the past. if you "quote" a post, and accidentally delete one of the quote open/close tags you get an error message. If you're using IE, when you go back you lose all the text you just typed in. Would be nice if this did a javascript validation (button or ajax based) before switching pages, or if it didnt refresh the page and lose all your changes on hitting teh back button..
have given up on posts several times when this has happened and just not bothered to start again