consider 2 categories:
#1 - 3 products.
#2 - 1 product.
navigating through cat#1 (flyage view of the products) works fine.
when viewing the single cat#2 product IE shows a "next product" link to a product in the cat#1!
just for comparison FF doesn't the $next_product variable is empty.
I'm using an SEF component but this happens even when it is off!
(don't blame it)
Couldn't duplicate, using Joomla 1.5.8 and VM 1.1.2 and default VM sample data