The one that comes to mind for me was in Cata.
Our guild had been 8/8 Heroic for quite some time, and I had finally gotten BiS everything... except the belt off of Zonozz. That thing just would not drop for me, and it never did. To this day I'm still bitter that of all the times I ran him (which was 19, btw) that I saw MAYBE one belt the entire time. It was the one piece I needed to be completely decked out for the expansion.