"An acquaintance of mine, 'Eric', was sitting in his driveway, on his shiny new Harley, polishing the gas tank. He lost his balance, fell over, and hit his head on the cement driveway (no helmet, as he wasn't going anywhere). He died 3 or 4 days later. You can suffer fatal head injuries in an impact as low as 4 mph."
I remember hearing just a few years ago about a famous drag racer who wrecked his little "pit bike" at very low speed...and died.