A script exploit on the site allows for HTML comments like scrolling coloured text.
However, once posted it disables all other comments on that video.
Its been done to a lot of videos, especially the most viewed ones.
I also got a pop up saying Justin Bieber was dead and to go to CNN.
Here is a video showing it in actionhttp://www.youtube.com/watch?v=ZgNjCmA7myE