You'll get a comment from a user that you've never heard of -- assuming that you allow comments from someone who is not on your friends list and the post isn't locked. And it'll say something innocuous like "I like your blog."
When you go to look at the poster, you'll find that they have no friends. And no posts. Today's spammer was created on February 25th of this year, so they clearly get generated in advance of use.
And then you'll get a barrage of anonymous spam posts to random posts in your LJ within the next day or two. Of course, you've screened anonymous posts, so you'll be the only one to see them. And you'll delete them as spam.
Lather, rinse, repeat.