This error crops up when there is already a shipping method selected when the user reaches the checkout. The error appears in a JavaScript alert box;
An AJAX HTTP error occurred.
HTTP Result Code: 500
Debugging information follows.
Path: /system/ajax
StatusText: Service unavailable (with message)
ResponseText: EntityMetadataWrapperException: Unable to get the data property type as the parent data structure is not set. in EntityStructureWrapper->getPropertyValue() (line 438 of /var/www/commerce/public_html/sites/all/modules/contrib/entity/includes/entity.wrapper.inc).
To replicate do one of two things;
Add a rule to automatically add a shipping method the first time an order is saved (my ideal scenario as it means the customer has a base shipping price). Proceed to checkout. Changes shipping service.
OR
Add something to basket, proceed to checkout, set a shipping service, close browser. Open browser and return to checkout, change shipping service.