It happens when I hit the post button.
It happens whether I've typed a single line and attempted to post or typed several paragraphs and attempted to post. The message text entry window clears and the error message is displayed.
It isn't happening in response to the enter/carriage return button on the keyboard - only the post button.
It's the WYSIWYG toolbar.
Assuming this post works then it isn't happening when I use Firefox.
Switch back to The Simple HTML toolbar, reload the post page and then revert back to the WYSIWYG toolbar and see if that helps.
Otherwise it seems there is a bug in TinyMCE somewhere or BH's implementation of it, but I wouldn't have a clue where.