Open up a wiki page
Click on 'edit'
Start making some changes
Browse to another page (can easily happen if your browser is set to open external links in same window)
The changes you made are now lost (some browsers will retrieve them if you press back, but not all browsers support this).
WHAT WAS EXPECTED
A warning dialog (with OK/Cancel buttons), to prevent accidental loss of edits
Suggestion: use 'window.onbeforeunload' to pop up a message and add 'onclick="window.onbeforeunload = null;"' to each of the 'save', 'cancel' and 'preview' buttons.