When the admin user has any of the custom-profile-fields with information, that field cannot be edited (Admin->Users->Accounts->User profile fields) anymore.
To reproduce, as admin:
1 - create any custom profile field.
2 - fulfill it in the admin user page
3 - go back to the custom profiles field and try to edit that custom field.
4- you'll get an: "That field shortname must be unique".
And 4) shouldn't happen. The field is correct.