DO NOT ENTER INTO
WHEN MOTOR IS RUNNING
Hazard signage on a vehicle recycling plant's pneumatic car crusher. Entrance by humans is discouraged due to the compression box's ability to reduce a car into a dense rectangular prism of twisted metal. This danger to life and limb exists only when the crusher's motor is ready to manipulate the hydraulic crushing surfaces. This fact combined with the fact that having the motor running is neccessary for said readiness (though possibly not sufficient) allow the safe qualification of the warning to apply only when the motor is running.
The device (but not the sign) was featured on the television program Junkyard Wars. However, the sign is visible in the bottom-left corner of the frame on the wide shots of the crusher in action.
Up there with "Do Not Eat" on the most unneccessary warnings of all-time, in the sense that all the warning serves is to prevent the removal of an undesirable element from the gene pool.