Back in my IBM days I was in a computer room with a raised floor. The customer was complaining that they had a machine people were getting shocked on. I figured out it was served by a surface raceway coming up the wall to a 5-15 that had obvious damage and checked the receptacle. Yup, 120v on the EGC compared to the bonded floor grid. It was a bolted fault in the raceway. The resistance of the EGC was high enough that it did not operate the OC device. You could cook a hot dog on the box under the floor that had the bad connection tho (loose EMT locknut, no green wire ground).
If we had been happy just repairing the bolted fault we would have only fixed half the problem.
