Interesting article!

Maybe two years back, one of my regular customers had me add a new circuit / receptacle to his data processing center. Since this was being added to an already packed room, I used MC, rather than my preferred pipe. When time came to make my connections at the panel, I went to attach the ground wire first .... and was quire surprised to see a spark! That day, I was able to replicate this several times.

On a follow-up visit, I was not able to repeat my experience.

I did the 'original' wiring of this place, and I installed their massive UPS system. I still have no idea what was happening ... but the article confirms that I was not imagining things, and gives me some idea of how to run this down.