If Drupal uses a node as startpage, and an admin deletes that user (ignoring that the user has ownership of nodes) the website has no frontpage and gives an error.
I suggest an extra check when a user gets deleted that displays a message like "If you deletes this owner and his/hers content, the website will have no frontpage. Are you sure you want to do that?" or perhaps even a level of security that prevents admin from deleting that particular user and his/hers content.