A day or two back I got quite a long comment on my rant blog. It seemed a bit odd. It was about the subject that I was referring to in that particular post, but didn't address it specifically.
Also, the comment seemed vaguely familiar. I felt that I had read it somewhere on another blog, or perhaps in a comment thread. So I did a search for a couple of the phrases in the comment and sure enough it was there in other places on the web.
I think the commentator might have been using it to try and lower my rankings for that particular post. Duplicate content in blog comments can hurt you, after all.
Or maybe he was just lazy...
In any case, it's definitely a sign that you should keep an eye on your comments, and delete all those that look spammy and/or duplicated.