I had a frustrating error for a while when using suPHP. It kept giving error 500 when enabling mod_suphp for .php files:
Directory /var/www is not owned by user
Of course, the directory was not owned by the user. The user belonged to a sub-directory only, the reason behind using suPHP.
It turned out to solve this the directory /var/www had to be owned by user root. That fixed the error.