Users can enter secondary e-mail addresses for their account. When doing so, it allows you to enter the same e-mail address twice as long as the case is different (eg JohnDoe@bitbucket.com v. email@example.com). Once logged in, the users gets a notification that one of the e-mail addresses has not been confirmed. When you go to the page to see which one it has an option to remove associated e-mail addresses. Since the two are essentially matching, the validation thinks you are trying to remove the validated e-mail and says that you cannot.
Issue #6728 closed