I have seen many cornices over decades of going to the tops of mountains and I am always watching for them, but the boundary of rock and ice is often hard to see. I can remember ones where it was hard to tell where the rock ended and the snow started since they are both capped in snow. One on top of Mt. Temple in early summer seemed particularly hazardous. Another one on the flank of Snow Dome was really a half-serac, just waiting to fall.
Good information on cornices can be found at the National Avalanche Center – Cornices
The tragedy on Mt. Harvey (which I have hiked several times) reminds us all to learn more about mountain conditions.