With this conf options combination, strange things happen.
$conf['start'] = 'index';
$conf['userewrite'] = '1';
When trying to access the main page (named 'index'), the page is redirected to unmeaningful page called like this: [[redirect:path_to_dokuwiki_doku.php]].
The 'path_to_dokuwiki' is taken from .htaccess' RewriteBase. The thing is, this happens only with start pages renamed after php files in dokuwiki's root directory (index.php, install.php, feed.php). With 'install' or 'feed', when you try to access them, the .htaccess shows them (this could't be understandable). With 'doku' start page, nothing special happens.
This behavior was observed on 2007-06-26 release and devel (snapshot 2007-07-02).