I found this interesting article in the opera blog. It is about CAPATCHAs and why they fail.
http://my.opera.com/portalnews/blog/2010/10/04/you-must-be-human-to-read-this-why-captchas-dont-always-work by Joseph D. Lien.
The Web tries to slow down spammers with CAPTCHAs but mainly succeeds in annoying everyone: Here are some neat facts about CAPTCHAs.
http://my.opera.com/portalnews/blog/2010/10/04/you-must-be-human-to-read-this-why-captchas-dont-always-work by Joseph D. Lien.