Best resource on this that I've found:
logged-in user + /user/register -> access denied
http://drupal.org/node/118498
There's no reason for this-- even user #1 gets access denied here, and users who could go to admin/user/user/create -- access denied might be technically correct, you can't re-register, but it's nonsensical and annoying.
Except where otherwise noted, content on this site is licensed under a Creative Commons Attribution 3.0 Unported License.