I worked in a shipyard building navy ships.
Had more than one hard hat get broke.
One I was running a chipping gun and someone came up behind me and hit me in the top of the head with a hammer.
Knocked me out.
It was a fiberglass hard hat and it left a spot about 2 inches you could push through with your fingers.
Yep. Standing up in low headroom after working in a crouch for a while and forgetting...OUCH!
My first full-time summer job was as an engineering techician at a mine. I didn't like wearing a hard hat, but I did it. One day I was doing some work in one of the surface plants, turned a corner, and smacked my head so hard on a pipe, I was flat on my back before I knew what had happened.
Didn't hurt a bit, though. In fact, I couldn't get up for a couple minutes because I was laughing so hard. Never complained about hard hats again.