You never added in your post,where this might be a dwelling or commercial building wiring.Believe it or not,might could have been helpful in this trouble shooting analysis.More info was needed on the part of customer,to you,to us.
In my opinion,
Just one of the things I'd check (maybe not the first),go to your Grounded Conductor bar(*neutral bar)
*unhook and *rehook grounded (*neutral)conductor branch wires(one at a time)...
I suspect this may narrow it down, to the culprit.
*It ain't of the supernatural..