* several user accounts with same email
* users with no openid
* users with no email (hack: gravatar set to settings.ANONYMOUS_USER_EMAIL)
* users with no screen name
* users with no email and no screen name (25% in homeschool)
* tag preferences are not stored explicitly (interesting/ignored)
  maybe they are in se.User.preferences_raw
  but the data there is not marked up and is kind of cryptic
* we don't have Community user. SE has one with id=-1
  this id may break the load script
  potential break places are anywhere where is X.get_user() call
  issues may happen with larger data sets where activity
  of user "Community" is somehow reflected in a way
  that load_stackexchange does not take care of