Improved script.js for Contact Form 7 reCAPTCHA Wordpress plugin. It shows an error message if the user fails to check the reCAPTCHA ("I am not a robot") box.

script.js

function contact_form_7_recaptcha_callback() {
    jQuery(function($){
        $('.g-recaptcha-explicit-id').each(function(){
            $(this).data('grecaptcha_id', grecaptcha.render($('#' + this.value)[0], {
                'sitekey' : contact_form_7_recaptcha_data.sitekey
            }));
        });
    });
};

jQuery(function($){
    $('.wpcf7').on('invalid.wpcf7 mailsent.wpcf7', function() {
        grecaptcha.reset($('.g-recaptcha-explicit-id', this).data('grecaptcha_id'));
    });

    $('.wpcf7').on('submit', function() {
        var response = grecaptcha.getResponse($('.g-recaptcha-explicit-id', this).data('grecaptcha_id'));
        if(!response) {
            $('#' + $('.g-recaptcha-explicit-id', this).val()).after('<span class="wpcf7-not-valid-tip">Please check the box.</span>');
        }
    });
});