Had a similar experience, luckily wearing helmet.
I've been riding two OneWheels for about 2.5 years and use it for my work commute. I've ridden at least 500+ miles and definitely know how to ride it and what push back feels like. With about 30% of battery left cruising along a mostly flat levy at my typical pace about 15mph on a ride I do very often all the sudden the nose dropped and I was dumped off. OneWheel + with about 350 miles on it. The board was still on and I connected the app to see the battery still had charge. It worked fine again without turning it off or anything and the app did not say anything about foot pads or anything, all appeared normal.
I had stopped wearing any padding/protection other than a helmet when running errands or commuting since I had not fallen except when messing around trying tricks or off road. I ripped my jeans and bloodied my hand, knee and hip. It did not hurt as much as just scare me that it could hurt a lot more if it happened again, in a worse location or when I wasn't able to roll with the fall as well. Now I'm much less comfortable riding and going slower just in case, which sucks because it is so much fun - but I just don't trust it enough right now.
Emailed FM and was told I had probably pushed through pushback, I've been riding enough to know what push back feels like and rarely ever hit pushback anymore - pretty sure I did not push through it. I will say I had just come down a small hill but and had been on the flat for ~15' when it stopped. It does get going down that small hill but I wasn't pushing it.
I agree with "George" - "From my experience it is a board defect. Either it should slow you down if you are driving to fast or slow down and stop if the battery is running low. This device is packed with electronics/senors and software and there should be no reason for it to suddenly stop AKA nosedive." Too many reports like this in the forums.