Years ago, I had two racks at FOH, both with Furman "power conditioner" rack units. I had one rack's Furman plugged into the other. I had my Mac laptop sitting on the one Rack that was chained to the other. When I rested my hand on the metal rack edge and touched my laptop, I got a mild but noticeable tingle on my fingers. I reconfigured my power so that that both racks were plugged into the quad box stringer coming from my main distro rather than a daisy chain and the issue subsided. I suspect there were some leaking MOVs causing noticeable ground voltage differential between the quad box and racks. Daisy chaining Furmans probably exacerbated the issue. I never do it anymore.