Removed the $userpage parameter from Namespace_Default::error_404(). It screwed up a couple plugins. (Thanks Mazza for discovering the issue)
# Used for testing mod_rewrite.
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule (.*) install.php?do=modrewrite_test&str=$1
</IfModule>