going back a few years I slid off on my RD250 and I slid under a marina van, I actually head butted the rear axle and my head did hit the tarmac a few times, my helmet was actually touching the floor when I made contact with the van, the RD missed me by an inch with the handlebars punching an hole into the fuel tank, I walked away with concussion and a seriously bruised leg which I think I was very lucky to escape with just that, I was at work the next day!!
so the moral of my incident was the helmet did save my life, but had the van been any lower by about 2mm the helmet would have killed me as I would have made contact with the angle iron tow bar fitted to the van.