Given a Page class that is modified to have a Text or Varchar field, set that field to some value in the CMS admin interface and "Save & Publish". Then, set that field's value to blank/empty and "Save and Publish".
In the database, the Page table will show that the custom field's value is NULL. However, Page_live will show the previous value of the custom field, not NULL.
Running against trunk (revision 80878).