Use different 404 page on front-end and back-end

Actually, it's quite the opposite ... 404_override may be a single value, but it points at what to look for in the current directory and then naturally falls back to the root one.

When you have application/controllers/Error404.php and application/controllers/admin/Error404.php, setting $route['404_override'] to 'Error404' will cause admin/Error404.php to be loaded if you visit /admin/non-existing-uri.

Did not know that, and I believe this is not explained in the user guide