I've seen on two different sites where entering a message that contained the site's url triggered the spam filter.
As an anonymous user I tried to add comment to:
Using name and email and simple message of http://treehouseagency.com ++
And on drupalmodules.com as a logged in user, jhr, I tried to post two links to spam on that site to the forum and got the captcha.
Seems like a url that's the same as the site shouldn't raise the spam flag.