I knew trying to integrate a plain text editor with a WYSIWYG editor simultaneously was going to be a nightmare :)

I'm going to look at this over the weekend, but you'll have to use the post editor for now. There are sophisticated regular expressions that parse through everything you submit to create the resulting post. Obviously, the expressions that handle plain text editing are broken.

Also, you can't submit HTML into the form whether you're using plain text or the post editor. All HTML is filtered so it doesn't render. If you're editing a post and the plain text editor doesn't convert the stored HTML into forum tags, submitting the HTML will make a mess of things.