This happens to me quite often in my O/Os thread. The system will tell you when you are over the character limit, but only once (in my experience) and never on an edit of a post. Other than that initial warning, which won't let you proceed until you get the post pared down, it doesn't warn. Nor does it show on preview as cutting off content from the end. It just does it when you post.
When I am doing long posts now I make a point to keep a copy of the original text in a word file (so that I always have a copy of the ending. But when you edit large files frequently as I do, you can go over the limit without even thinking about it. I had to reconstruct the ends of two of my idea posts just last week because they had gotten too long and gotten cut of at the end.
It's not an elegant work around, but it helps I working with the system as is.
Another possibility is trial by PM. PMs and posts seem to have the same character limit. So when I'm not sure about the length of a post. I'll sometimes simply PM it to myself to see what it does. That requires keeping a full text copy. I usually make a copy with full formatting just before hitting the send or post button.
