One word: Internet Explorer 6. This includes a rewrite of $paths->parseAdminTree() that encodes to JSON instead of manually generating JS, so good-bye to stupid parser problems I hope.
# 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>