1. Go to Special:Preferences.
2. Do not change anything.
3. Navigate to another link (for eg. click on the Watchlist link in personal tools)
- Actual: Popup appears asking for confirmation "This page is asking you to confirm that you want to leave - data you have entered may not be saved.".
- Expected: It shouldn't appear if nothing was changed.
Browser/OS: Firefox 43, Ubuntu
Seems to have been caused by https://gerrit.wikimedia.org/r/#/c/255652/