Thanks for the clarification. Luckily BNC connectors are rare nowadays. It happened at a friends house. I lost contact but I'm pretty sure he's still alive but maybe their house is still dangerous.
This can manifest anywhere that has grounding issues and electrical wires that are not isolated from human touch, not just BNC.
For example, in conventional Ethernet, you have to ground the shielding for EMF reasons, but you usually ground only one of the shieldings on both sides of a cable - conventionally, at least in Germany, in the distribution rack of the patchpanel, and if you connect two distribution racks/rooms, on the "upstream" side - but you never ground both ends as that can not just cause ground loops but especially very weird issues where fault current suddenly doesn't pass back to the AC distribution panel on cables with the proper wire gauge but through the (very tiny) gauge of a network cable shielding. That's a perfect recipe for invisible fires, and even assuming there is no fire, some points of the effective "micro grid" can be at a high enough potential to shock someone.
Neutral connection losses/resistances in multi-phase grids are even worse because these aren't just a threat under fault conditions - here, "normal" operating return current makes its way back to the actual neutral/ground via whatever path it can, and that may even include water pipes [1].
