This was recently brought to my attention through a Twitter posting.
Clickjacking is a technique where a web developer can trick you into clicking on something that you don’t mean to click on. In this example, it tricks someone into updating their twitter account without their knowledge.
Check out the video (done with the totally amazing Screenflow software), and read up on it on James Podolsey’s blog.
If you use firefox and want to block this sort of thing, grab the Noscript addon.