If you paste a Text from a HTML Page (or Word) with a <h3> tag or enter a <h3> tag in the Wiki-Editor and open the WYSIWYG Editor and safe it afterwards, the whole text gets lost.
I had to comment/remove the line
in wiki_editor/plugins/core.js to make it work. I don't see any reason for this line, if you have it just for h3, there is no heading conversation or anything, would be a nice feature though. Anyway, I think like this it's just a strange bug...