There's a worm on Facebook posing as a Facebook application. It's called the Error Check System and what it's supposed to do it notify you when your friends get an error when trying to view your Facebook site. It sends a messages saying "[Facebook User's Name] has faced some errors when checking your profile View The Errors Message".
When you sign up for this Facebook application so that you can see the error, all your friends are also sent a message.No one's sure just yet what the Error Check System worm does. There are suspicions that it may be gathering information about you and your friends.
If you have unwittingly added the Error Check System application to your Facebook site, don't despair, there is a way to get rid of it. About.com's own guide to Instant Messaging, Brandon De Hoyos, has the fix you need to get rid of the Error Check System worm.