To recreate this issue:

• Configure a rich text field in a detail subapp (contacts app for example)
• Open another app like the pages app
• Switch back and forth between the detail subapp and the other app
• You should get something like seen in the screenshot

I will say it doesn't happen every time but it does happen if you try it a few times. You should stumble over one. I also tried with Firefox and Opera and could not reproduce it but I could see that ckeditor did seen to "stick". You can see the subapp close, then briefly you see the ckeditor, then you see that other app you open but the ckeditor does get covered up by the opening app. So this may be an issue in other browsers as well but I just wasn't able to reproduce it fully.