Reports come from WebmasterWorld that Google recently stopped obeying the noindex meta tag; <meta name="robots" content="noindex">. The tag is suppose to prevent Google from displaying your pages in the search index.
WebmasterWorld senior member, g1smd, explains that this is some sort of Google bug;
Such a page should never appear in search results, ever. Well, now they do, and in very large numbers; all (so far) marked as Supplemental Results, and with cache dates from a year ago.
It appears that something in their system is forgetting to check the index/noindex status of the pages in their database and is showing them all in the SERPs whatever their status.
I first noticed this yesterday; but found it on some searches that I have not done for several months. I have no idea how long this bug has been showing up... it could be several months.
tedster, WebmasterWorld Administrator confirms g1smd statement, this certainly is an error/bug and not just some intentional change.
As tedster points out, the noindex meta tag is an invaluable tool to help Webmasters prevent duplicate content issues. If Google accidently took that away, there can be serious issues in the short-term future for those Webmasters that heavily used the noindex meta tag as a solution.
Forum discussion at WebmasterWorld.