I just had a site maintainer create a url alias for a blog using:

alias: blog/test
for url: blog/8

This broke logging into the admin - right after a user logged in they were redirected to page not found. This happened to all users, including the admin user. So subsequently did the correct way to do this using Token and Pathauto but thought I'd mention in case someone else runs into this. Probably URL Alias UI should be changed to prevent this. Using Drupal 6.16.