Myspace CSRF and XSS Hack - Chris Shiflett:

This attack seems pretty harmless (I'd rather not discuss ethical concerns), but it demonstrates something very powerful - a combination of XSS and CSRF. If your site has XSS vulnerabilities, they can be used to launch much more effective CSRF attacks. Rather than only a small percentage of people being affected, everyone is, because the attacker is guaranteed that all victims have an established relationship with the target site, yours.

(Via Planet PHP.)