That cloud looked exactly like it would if a chunk of tile had been shattered. I've busted a scrap piece, and it's like really lightweight chalk. Closest thing I've ever seen to it was that horrible dried ice cream they sell at NASA souvenir shops. In fact, I've long suspected that the "ice cream" was actually scrap tile pieces.

There have been many tile breakages through various missions, most of them just through the hard tile surface to the lightweight material underneath.

I think what happened with Columbia is that a tile finally broke just the wrong way or in just the worst spot to create a burnthrough. Maybe a tile finally broke through to the aluminum or felt underneath, or maybe the breakthrough was at a point where there was a very hot shock impingement at reentry. Maybe had the broken tile been a few feet to either side, they would have made it. We'll know more later.