Thanks, @DA RAIDERS, @mr furley , and @cosjobs -- my best guess so far is that the forums fire off some sort of javascript to clear out the saved contents of your editor after the reply is posted, and because of the size of this thread, that script is either timing out or not getting fired.
I know that the rich text editor that Invision uses saves its contents to your browser's local storage to reduce the chance of you losing a long post by accident. But it is supposed to clear the editor out once the server confirms to the client that you've successfully replied.
I'll check with In