There's a YouTube commenting bug that allows anyone to easily inject redirects/code/... into any comment. The script kiddies at 4Chan are already exploiting it in an attack on Justin Bieber videos, but there are many harmful redirects as well.

It would be possible to become a victim of this type of attack from a youtube link on boards such as this one so best avoid everything 'youtube' till they sort out their sloppy code.

Apparently YouTube can't fix it quickly so they've turned off their commenting system for now.