1 - register like this http://ahmed.d.pr/19Poz
2 - register a new user like this http://ahmed.d.pr/1eExL
3 - choose permalink >> Post name >> /%postname%/
4 - the link in the browser must be like this author/name/
5 - but in this case will not work and give 404 page
6 - so in this case i’m must change the permalink for the users to ?author=id and this not a good for the seo and for the users .
Read more here: problem in the author permalink