Hi
When I add an embed to the page the editor adds "Default preview for : Embed". If I remove this text and switch between html and normal view the text re-appears. Tested with chrome and IE11 on your demo page.
To recreate:
1: Goto http://www.richtexteditor.com/demo/
2: Paste <embed id="wiggle"></embed> into html view.
3: Switch to normal view and submit.
Can you stop this from appearing please.