Responsive Theme Menu Error?

I have used a largely un-customised Responsive Theme layout for edinburghfabrics.co.uk – but I have just added the plugin called WP Lightbox 2 and it seems to have a conflict.

When I click on a picture using Google Chrome or Firefox to display the larger ‘lightbox’ version, for example on the page for Fabrics, then close and return to the page, a strange drop-down menu is appearing underneath the standard (shaded black) menu links. The new drop-down menu is an unwanted side-effect.

Is this a bug or a Javascript conflict perhaps?