On front-end login form, keep user from being re-directed to an admin login page when a field fails

restrict-failed-login.php

/**
 * Keep a failed login attempt on the same page instead of re-directing to backend
 * @access  public
 * @since   0.1.0
 */
add_action( 'wp_login_failed', 'my_front_end_login_fail' );  // hook failed login
function my_front_end_login_fail( $username ) {
   $referrer = $_SERVER['HTTP_REFERER'];  // where did the post submission come from?

   if ( !empty($referrer) && !strstr($referrer,'wp-login') && !strstr($referrer,'wp-admin') ) {

	  $pos = strpos($referrer, '?login=failed');

		if($pos === false) {
		 	wp_redirect( $referrer . '?login=failed' );  // let's append some information (login=failed) to the URL for the theme to use
		}
		else {
			wp_redirect( $referrer );  // already appeneded redirect back
		}

      exit;
   }
}