Last modified: 2009-09-28 17:35:00 UTC
Open a page for editing, make a change, try to close the tab. A message will pop up:
"Leaving this page may cause you to lose any changes you have made.
You can disable this warning in the "Editing" section of your preferences."
The second line should either be omitted for anons, or tell 'em to register to have access to all those cute prefs.
Tweaked message in r57019.