I add the HTML code below to the CKEditor source editor:

<p><a href="/link"><em>text</em></a></p>

If I switch to the visual editor and back to the HTML editor, or if I save it I get the following in the source:

<p><a href="/link"><em>text</em></em></a></p>

Doing this 5 times results the code below:

<p><a href="/link"><em>text</em></em></em></em></em></em></a></p>

<p><em><em><em><em>&nbsp;</em></em></em></em></em></em></em></em></em></em></p>

 

I have the latest CKEditor 2.1.3, and using a default DNN skin.

I don't know if this is related to this but I see the error below in the console:

jquery.js?cdv=108:8526 

GET http://htmlg.com/Resources/Shared/scripts/widgets.js?_=1446547884990 404 (Not Found)

Leupold, I think you mean Admin > Site settings > Advanced settings > Enable popups because I can't find anything related to "widgets" in site settings. I tried this both active and disabled but it didn't make any difference.

I disabled skin widgets, but that didn't help.
I see this is a common bug with CKEditor because I have the same issue on another site.

Simply add this code to the source and switch between Design and HTML a couple times:

Test