Are you locking out ALL sources of energy?

On December 28, 2006, Employee #1, a temporary employee, was working as an oil well pumper. He was sent to investigate squeaking belts on well number 11, a Lufkin Pumping unit. Employee #1 did not lockout the pumping unit prior to entering the fenced area, and the counter weight of the pumping unit struck Employee #1 in the head, killing him.
Source Osha.gov