I had an incident last weekend that was really quite unexpected. While routing interconnect cables from my Dodd MLP to a R2R deck (with all components in the system powered off), the ground shell of one of the IC's RCA plugs touched a screw in the bottom of the MLP's external power supply. Arc...POW! Power supply dead.
It's on its way to Gary's for repairs, but I thought I'd share this to save someone else a bit of grief. As I mentioned, all the components were powered off--but the external power supply always has power to it. And (EEs jump right in), there was apparently a large enough potential between PS chassis ground and preamp chassis ground to create the arc. The screw that was touched and the RCA are both pitted from the event.
The only way to avoid this seems to be to unplug the external power supply any time you are making connections. At least that's the methodology I'll employ in the future.