Spam Comments

I have been getting a few
"This a good post. I like you see my post too."
comments with dubious URLs attached, recently. I usually delete them without even looking, having once been directed to an Indonesian online sales site.

Recently, my mum complained that she had a received a message saying her comment had been deleted by the blog administrator. On checking the post, I saw that I had indeed deleted a spam comment on that entry, but was not aware of anything coming from my mum. It made me wonder whether spammers are hacking in and piggybacking on people's comments? Has anyone heard of this before? Has anyone of my regular commenters had a comment deleted by me and wondered why? (and never come back again so won't be reading this anyway....)

I'd be interested to hear your experiences.