When I click on reply for this post, the fonts for the edit box and everything else change. Once I finished the reply and returned to the thread, they returned to normal. It's probably related to the rendering in the preview window with the post I was replying to. An open formatting code, perhaps?
2. There is an unclosed [font size="4"] tag in that post.
It spills over to what follows in reply pages, but is probably overridden by style declarations for the elements on regular pages. You can do the same thing with your OP if you edit in an unclosed [font] tag.