I think this a bug, I have set the sort order of a number of products so they are in the order I want on the product listing page, however when I click on a product and and use the Previous / Next buttons on the product info page I find the order of the product is not how I see them on the product listing. Looking at the order the previous / next is using I can see the order is based on the Product Name, so proucts with a Z in the name will be the last product in the previous next even if you have set it with a high sort order.
I hope what I just tried to explain makes sence, I will try to explain better if not.
Regards
Mark



