How can web server configuration files (e.g., Apache .htaccess or Nginx rules) lead to a 404 on localhost?

Misconfigurations in web server files are a common cause of 404s. An Apache `.htaccess` file or Nginx server block (within `nginx.conf`) might contain incorrect `RewriteRule` directives that redirect requests to non-existent paths, deny access to certain directories, or fail to correctly route requests to your application's entry point (e.g., `index.php` or `index.html` for SPAs). Ensure `AllowOverride` is enabled for `.htaccess` in Apache's main config, and that your Nginx `location` blocks correctly define root directories and proxy passes for dynamic content.