I actually have seen it happen once time. But it has happened a few times here in central Ohio. The one I saw the bed just ripped some wires across the road. He had left the PTO engaged and the dump lever got activated as he was driving. Not loaded.

Another one, the guy was on the freeway, bed raised up for some reason. Caught a bridge as he went under it, slammed the cab up into the underside of the bridge, killed the driver.