It happened tonight that I went checking back through this blog to find something related to Bill Bryson and design flaws. To my surprise there was a relatively new comment on the post, even though the post itself dates from September 2005 (it's one of the earliest ones on this blog, in fact). I was possibly even more surprised to find it was what you might call a 'spam comment' - in other words, if a human had written it, he or she had done so some time ago, and had then sent out the comment to any sites that would accept it. There must be something about this blog that allows those sorts of comments to be picked up, at times.
I scrolled through the other posts around that period and sure enough there were scattered comments on different posts advertising sites where you could buy diet drugs, or cheap insurance and other such stuff. They mostly seemed to be in the 2005-2007 period, so perhaps the bots that send these things are geared towards older posts so that they're less visible to the blog owners.
Interestingly enough on one of my other blogs, The Daily Writer, I get notifications via email regularly that 'someone' has 'commented' on a post - but these are newer posts. One in particular has suffered constant comments of this kind. Normally by the time I check them out they've been removed - presumably by some program that Blogger/Google runs. I wonder why that doesn't work on this blog, which is run by Blogger as well?