ReferenceError: switchEditors is not defined post.php:1
The above is what I saw in the browsers' JavaScript Console (control-shift-J in Chrome and Firefox).
Make the WordPress user experiencing the error an administrator rather than an editor (or other non-admin).
When one tries to edit a page, the page is blank and the editing buttons are gone. Switching between text (HTML) and visual may cause this blanking-out to happen.
There are a handful of other solutions I've seen on the web (2 - 3 other solutions in ~7 articles). I did not try the others because I was not able to reproduce the bug for this write-up. There may be several paths to the same error message with different solutions, or perhaps those other solutions will work.
This was in WordPress version 3.9.1, observed 2014/06/29.
I suspect that several conditions have to happen before you see the bug. I reproduced the bug immediately in June, but I tried for hours on November 10, 2014 with no luck. When I downloaded wordpress-3.9.1.tar.gz, I noticed that in the Dashboard, the version showed 3.9.2. That ambiguity might be part of my problem with reproduction.
It would not take much persuasion for me to fix the bug. I think I could do it in a few hours. First I have to be able to reproduce it (again). If anyone can give me a scenario, let me know--see my resume for contact info.
edited page goes blank, cannot edit.