when digg released their new comment system about 2 weeks ago they also made an update to their url structures to bring them in line with a previous update where they had implemented a universal taxonomy. The universal taxonomy allows you to submit any type of content (news, pictures, videos) to any category (technology, world and business, science, and so on) and then sort by media type in the upcoming and front page sections for any subcategory.
so, for example, you can browse upcoming as:
http://digg.com/[media type or 'all']/upcoming/most
or browse the front page as:
http://digg.com/[media type, 'all', or leave blank]
the problem, was that they had implemented this for the overall upcoming section and the front page but hadn’t actually implemented it for upcoming sections for different categories or subcategories and you could only browse upcoming subcategories as follows:
http://digg.com/[category or subcategory]/upcoming/most
however with this update they have applied the universal taxonomy to those as well, transforming the link above to:
http://digg.com/[media type or 'all']/[category or subcategory]/upcoming/most
and what they failed to do was redirect the older structures to the newer ones. in fact, when i first encountered this problem i thought that the site was actually down or having problems (because i was using the older url structures not knowing that they had changed and not finding out because they weren’t redirecting).
now i know only a little about how search works and how the integrity and value of different pages on a site work but i don’t imagine this being good for them at all. so, note to digg: use redirects.