I got the following message by e-mail from a visitor today:
Hi there - I was just trying to comment on a blog post on your site when I was confronted by this message:
"Error: This file cannot be used on its own."
It is in/on the "wp-comments-post.php" file after a comment submission..
I'm not sure what's going on, but I think it has to do with one of the spam plugins I'm using, the two are Bad Behavior and lr2spam.