Depending on the browser being used, they can be really finicky or just pass over the mistake.
Developer's tools show several errors but the W3 Validator is only concerned with an open end tag for a ul.
I believe the Validator itself is confused by the code presented in the "handheld menu" portion of your main page. That's some really gnarly code.
After breaking it out and putting it in a better format (tabbing the lines), it looks like
Code:
<!--bof handheld menu display-->

<script src="includes/templates/responsive_apparel_boutique/jscript/script.js" type="text/javascript">
</script>


<div id="container">
    <a class="toggleMenu" href="#">
        <img src="includes/templates/responsive_apparel_boutique/images/mmenu.png" alt="m menu image" />
    </a>
    <a class="m-home"></a><a href="http://corset.ie/">
        <img src="includes/templates/responsive_apparel_boutique/images/mhome.png" alt=" m home" />
    </a>
    <a class="m-cart" href="http://corset.ie/shopping_cart">
        <img src="includes/templates/responsive_apparel_boutique/images/mcart.png" alt="m cart" />
    </a>
        <ul class="nav">
           <li>
               <a href="http://corset.ie/shopping_cart">Shopping Cart - 0 item(s) - €0.00</a>
           </li>
            <li>
                <a href="https://corset.ie/login">Log In</a>
            </li>
           <li class="test">
               <a href="#">Corsets</a>
                    <ul class="level2">
                        <li class="submenu">
                            <a href="http://corset.ie/clearance-items">Sample Sale</a>
                                <ul class="level3">
                                    <li>
                                        <a href="http://corset.ie/-c-57">Basque Style Corset</a>
                                    </li>
                                    <li>
                                        <a href="http://corset.ie/-c-61">Bridal Corset</a>
                                    </li>
                                    <li>
                                        <a href="http://corset.ie/-c-58">Burlesque Corsets</a>
                                    </li>
                                <li>
                                        <a href="http://corset.ie/clearance-items/accessories">Hand Ruched Taffeta Skirt</a>
                                    </li>
                                    <li>
                                        <a href="http://corset.ie/clearance-items/hourglass-underbust-corset">Hourglass Underbust Corset</a>
                                    </li>
                                    <li>
                                        <a href="http://corset.ie/-c-56">Old Gold Satin Overbust Corset</a>
                                    </li>
                                    <li>
                                        <a href="http://corset.ie/-c-59">Overbust Corset</a>
                                    </li>
                                    <li>
                                        <a href="http://corset.ie/-c-60">Taffeta Corset</a>
                                    </li>
                                </ul>
                        </li>
                        <li>
                            <a href="http://corset.ie/overbust-corsets">Overbust Corsets</a>
                        </li>
                        <li>
                            <a href="http://corset.ie/underbust-corsets">Underbust Corsets</a>
                        </li>
                        <li>
                            <a href="http://corset.ie/bridal-corsets">Bridal Corsets</a>
                        </li>
                        <li>
                            <a href="http://corset.ie/waist-training">Waist Training</a>
                        </li>
                        <li>
                        <a href="http://corset.ie/leather-corsets">Leather Corsets</a>
                        </li>
                        <li>
                            <a href="http://corset.ie/mens-corsets">Mens Corsets</a>
                        </li>
                        <li>
                            <a href="http://corset.ie/lingerie">Lingerie</a>
                        </li>
                        <li>
                            <a href="http://corset.ie/jane-woolrich-lingerie">Jane Woolrich Lingerie</a>
                        </li>
                        <li>
                            <a href="http://corset.ie/accessories">Accessories</a>
                        </li>
                        <li>
                            <a href="http://corset.ie/gift-certificates">Gift Certificates</a>
                        </li>
                        <li>
                            <a href="http://corset.ie/gift-box">Gift Box</a>
                        </li>
                    </ul>                        
            </li>
            <li>
                <a href="#">Quick Links</a>
                    <ul>
                        <li>
                            <a href="http://corset.ie/products_new">New Products</a>
                        </li>
                        <li>
                            <a href="http://corset.ie/featured_products">Featured Products</a>
                        </li>
                        <li>
                            <a href="http://corset.ie/products_all">All Products</a>
                        </li>
                        <li>
                            <a href="http://corset.ie/specials">Specials</a>
                        </li>
                    </ul>
            </li>
            <li>
                <a href="#">Information</a>
                    <ul>
                        <li>
                            <a href="#">Customer Service</a>
                                <ul>
                                    <li>
                                        <a href="http://corset.ie/about_us">About Us</a>
                                    </li>
                                    <li>
                                        <a href="https://corset.ie/login">Log In</a>
                                    </li>
                                    <li>
                                        <a href="https://corset.ie/create_account">Create Account</a>
                                    </li>
                                    <li>
                                        <a href="http://corset.ie/shippinginfo">Postage &amp; Returns</a>
                                    </li>
                                    <li>
                                        <a href="http://corset.ie/privacy">Privacy Notice</a>
                                    </li>
                                    <li>
                                        <a href="http://corset.ie/conditions">Conditions of Use</a>
                                    </li>
                                </ul>
                        </li>
                        <li>
                            <a href="#">General Info</a>
                                <ul>
                                    <li>
                                        <a href="http://corset.ie/site_map">Site Map</a>
                                    </li>
                                    <li>
                                        <a href="http://corset.ie/gv_faq">Gift Certificate FAQ</a>
                                    </li>
                                    <li>
                                        <a href="http://corset.ie/discount_coupon">Discount Coupons</a>
                                    </li>
                                    <li>
                                        <a href="http://corset.ie/unsubscribe">Newsletter Unsubscribe</a>
                                    </li>
                                </ul>
                        </li>
                        <li>
                            <a href="#">Useful Links</a>
                                <ul>
                               </ul>
                        </li>
                    </ul>
            </li>
            <li> 
                <a href="#">Search</a>
                    <ul class="mobile-search">
                        <li>
                        </li>
                    </ul>
            </li>
            <li>
                <a href="http://corset.ie/contact_us">Contact Us</a>
            </li>
        </ul>
<div>


<!--eof handheld menu display-->
I've highlighted in red what probably needs to come out to satisfy the Validator.

The worst offender is probably the ul without li.

After a fix, it may work in all areas but, I'm still wondering why the handheld menu display is even showing on the desktop version.