I have a multisite that uses Jigoshop and was receiving this error message when something was added to the cart:
Fatal error: jigoshop_cart::calculate_cart_total() [jigoshop-cart.calculate-cart-total]: The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "jigoshop_product_variation" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in /home/bjewelry/public_html/wp-content/plugins/jigoshop/classes/jigoshop_cart.class.php on line 507
It works on IE but not the other browser such as google and opera. Plus I was not able to get back to the site. If I tried to access in one of those broswers it was not able to locate the site.
I discovered it was your plugin after comparing it to the test site and noticing I hadn't installed your plugin on the test site. Once I deactivated your plugin the site was back to normal.
I am using the latest Jigoshop verision 1.7.3.