missing from google's index

A few days ago, one of my posts went missing from Google's index.
Which was rather strange because I had checked a few hours after publishing it that it was turning up in Google searches and Google Analytics had already started showing me hits on my blog because of that post.

After some digging around I found out that pages get taken out if a spam site links to them and then those pages are inspected in detail before putting them back in the index. And, if the page links to a spam site, it is taken out immediately and kept out. The latter turned out to be the cause of the post going missing. A spam site had left a "backlink" to my post and the post was inadvertently showing the link to the spam site. So I went and disabled all backlinks and the next day my post was showing up again!

The curious thing is that links in comments and backlinks/trackbacks usually have a rel=nofollow set. So I can't imagine why that backlink would have had any affect, but mysterious are the ways of Google. ;-)