If the field contains some value already (default value) and you change it, and then undo those changes, getting the value of the field to the old one, it will still prompt about navigating away.
Steps to reproduce:
1] Go to browse tab of a table.
2] Click on "edit" of a row (not inline editing).
3] On Edit page, the fields will be populated with current values. Change any field value.
4] Change that field again so that it is same as old value which was populated by default.
5] Click on some link, try to navigate away from the page.