Steps to reproduce

1. Enable the preference "Warn me when I leave an edit page with unsaved changes" at Special:Preferences#mw-prefsection-editing
2. Go to https://en.wikipedia.org/wiki/Special:MyPage/common.js?action=edit
3. Code something cool! (ok, for now just type "test" :-)
4. Try to close the page

Expected result:
There should be a warning such as "Leaving this page may cause you to lose any changes you have made. If you are logged in, you can disable this warning in the "Editing" section of your preferences."

Actual result:
The page is closed without warning and you lose your work!

Browsers

• Google Chrome 32
• Firefox 26