Decades ago, I was removing the driveshaft from my truck. Almost flat driveway, and I'd never had it roll before. But this time, it did. Rolled all the way to the street, very slowly, with me underneath the truck.
3 of the bolts removed, and one loose. The driveshaft oscillated around that one loose bolt, with my arm being jammed and cut up on each revolution.
That was very stupid! Took months to heal.