Order with no colour option selected

WC version: 2.3.11

Ever since the latest update, I have been receiving orders that do not have a colour attribute selected. This only happens on orders that have two or more products in the cart (Although the shop only has one product, there are attributes for size, colour and of course quantity). Always one of the products in the cart will have the colour attribute but the second one does not.

I cannot reproduce the error but is is happening consistently with live customers.
Email order: https://www.dropbox.com/s/9t7fr39wrke0jfx/Screen%20Shot%202015-06-29%20at%2012.18.58.png?dl=0

The website in question: wrapandstick.com
I though it may be a js problem butI cannot see any errors in firebug or chrome inspector. If setting the add to cart button to display:visible; in the web inspector, the product is not added to the cart (kit bag) until a colour is choose so I dont know how this error is occurring.

Any ideas on how to track this down?

Thanks in advance.

https://wordpress.org/plugins/woocommerce/