We are running 3.9.1, and haven't updated any plugins or our theme since the user started seeing an issue with a particular page. If the user opened up the page, nothing showed in the visual editor, unless they selected content, etc. If the user clicked to switch the editor, they had a 'switchEditors is not defined' error.
What is stranger - anyone else could edit this page just fine. Also, no matter what machine this person used, it showed the same issue, but only for them. User tried 3 browsers on one computer, 2 on another, and 1 on yet another machine.
Copying and pasting content from one page to another worked fine - so the content wasn't an issue (it appeared) either.
How could something like this follow a users account, and not the browser?