What is called Supplemental Page Issues in search engine?

"Supplemental Hell" to webmasters, the subject has been lurking in places like Webmaster world for over a year, however it was the major shake-up in late February (coinciding with the ongoing BigDaddy rollout) that lastly led to all hell breaking loose in the webmaster community. You may be aware that Google has two indexes: the main index, which is the one you make out when you search; and the Supplemental index, a graveyard where old, erroneous and obsolete web pages are laid to rest (among others). Nobody's disputing the want for a Supplemental index, it does indeed give a worthy cause. However when you're buried alive, it's another story! Which is accurately what's been happening: active, recent, and clean pages have been showing up in the Supplemental index. The exact nature of the issue is not unclear, nor has a ordinary causing leading to it been determined.