The Error-- Regardless of where the Duplicant Motion Sensor is placed, no duplicant who is working at the Rock Crusher will ever be detected by it.
Why it's Important--
If you want to get the production bonus awarded by placing a light bulb there when using the Rock Crusher, you will be unable to limit power draw used by the light such that it only turns on when a duplicant is working the machine (i.e. it's impossible to make it so the light comes on only when it is needed)..
Steps to Recreate it--
1) Build a rock crusher & surround it with Duplicant Motion Sensors. Put supplies near it and start it making some valid recipe(s).
2) Switch to Automation View.
3) Permit/Force a duplicant to use the Rock Crusher, ensure no other duplicants enter into range of any of the sensors.
4) Observe none of the motion sensors will output a green signal when the duplicant is working at the Rock Crusher punching the red button.