One panel box was miswired, a type of mistake I don't understand could have been made in the first place. Had it been energized, it would have made a dead short. That one was easy. The other taxed my memory. Finally found it in an dated book I have, but the code has changed since then. It, too, was miswired but wasn't quite the obvious mistake that the first one was. Not being a licensed electrician, I won't comment further on these.
The first was, I think, due to a broken bonding strap on an outlet. Have dealt with that exactly once, and it was a pain to find. All I know is that the electrician replaced an outlet in a non-obvious location and it corrected the issue. The symptoms described to me had me thinking it was a bonding strap problem or a junction problem. Either way, I recommended calling an electrician and leaving that breaker off to be on the safe side, which they did.
It takes only about a amp of power through the heart to stop it. I knew someone online who died working on an project (I think electronic) from that very thing. One hand only is a good policy, but it pass through the heart in other ways. Should point out to anyone reading that the rubber boots or overshoes should be rated for that sort of work and voltage, as some rubber products incorporate carbon and aren't as good an insulator as we might think. Same for rubber gloves. Voltage is a difference in potential, and if the source potential is high enough, it will flow up one leg and down the other just from the difference in voltages between the two.
I work for an electric utility, and always use a device like a multimeter or voltage sensor to check before working on anything. Never try to check for voltage using the back of your hand, thinking the jolt will knock it away. At work we call that an open casket funeral. Touching it the other way we call a closed casket funeral.