I had this issue pop up for me in WordPress 3.7.1. There are several threads on this problem on the forums. I believe I found a trigger which can be reproduced. If any core developers feel like taking a look .. it can probably be removed from WordPress.
Somehow my site url and home url had been blanked. Probably happened during search and replace on database migrations. With these fields blank, the en.js file problem appeared. After setting those options again, the problem went away.