Sumner-
I actually had this happen to me on a post yesterday or the day before. Just as you describe, no rhyme or reason. I ended up copying the body of the message, refreshing the page, pasting it back in and it took. I did file a ticket on it with engineering. It only happened to me one time. We made a bunch of changes to the editor with this last release and created a basic BBCode editor as well. Let me know if it happens again so I can update the ticket. Sorry about that.