There was this 12-bit analog-to-digital converter (ADC) which was set up for testing in a fixture that this fellow had assembled in a 19-inch rack that was maybe seven feet tall.
First, let's look at some numbers:
The ideal mV steps of this ADC were one part in the twelfth power of two, take away one (for zero to +5V exactly). That comes to 1/(212 - 1) or 1/4095. At the ADC's full scale voltage of five volts, that comes to a nominal resolution of 5/4095 volts which is very nearly 1.221 mV. The ADC tests were failing their test limits and I was asked to find the reason why which did not please the fellow who had set up that rack. I began the process anyway.
Using a handheld digital multimeter, I found offset voltages in the "ground" wiring that were as high as 50 mV, way too high to be tolerated while trying to measure values like 1.22 mV. However, when I pointed this problem out, the fellow's answer was memorably stupid right down to the present day:
"Ground is ground."
This guy's concept of ground reminded me of a probably apocryphal story I read many years ago about a television repairman who answered a call to a customer's home.
The customer was this very elegant, elderly lady who had a crystal set as her radio and the crystal set had stopped working. The repairman wasn't expecting to find something like that in this day and age, but a service call was a service call so he set about looking for the problem and sure enough, he found it.
"What did you find?" his customer asked. "I found the problem.", he replied. "Your ground is no good."
She glowered at him.
"My ground is no good? I'll have you know, young man, that my garden soil is the best in town!"
I think these two ground experts had something in common.