Troubleshooting 101

A long time ago I was on a job interview after passing the written test. The written test had such questions as: "Design a 24VDC power supply with supply voltage of 110VAC in with less than .5v ripple voltage. Show all work". The interviewer first thing opened up a desk drawer and handed me this simple object (yep it is the same one from long ago):

A flashlight. He told me it was not working. I turned it on and it DID work. I was told later that if you started to take it apart without verifying the complaint you failed this question.

So many in the field want to discover something. Case in point 25 plus years later after that lesson we have this scenario.  Operator says he is getting incorrect readings on a machine display. OK, fair enough. Looking at the display several indicators are obviously wrong. Time to go to a 19 year old PLC (programmable logic controller) for a look see.

Ah ha! Closer inspection revealed this:

Two identical analog in cards. The one on the left shows from the LED's two things. A.- It's not ready and B.- not active. 

Some would have already pulled out a voltmeter, loop simulator, ect. Check the obvious first!