You never know, even with a hoist. My Uncle was a mechanic for years. He was working underneath a truck that was on a hoist. The truck was only a few feet off of the ground and he was sitting flat on the ground. Suddenly, the hoist lost hydraulic pressure and bend him over double. In was under there for an hour until someone came into the shop. In the meantime, he had suffered a heart attack. He recovered but had to retire.