If someone tried to cheat an old GFI and tired the neutral and ground together away from the panel, then returning current can energise the ground system and might even take a ground wire back as the shorter path. Also if someone made a neutral ground reverse the ground system takes back the returning current and anything bare gets heated up, I think some kid got killed in a ground neutral reverse in Australia on some 230V when he turned on the hose and got between two grounds.
Bonding everything (across water meters, water softener pipes, hot water heater pipes, run a bond to the gas line and at the furnace) works to equalize the ground, you get hit when there's a difference. Also the white wire is your first to make and last to break, if you get hit, the neutral takes current back and you might only get some of it when you get hit to the ground , (where the neutrals and grounds meet at the main panel only) if there's current and you get between the hot and the ground, or between two neutral wired taking current back to the panel, you become the path and it is much worse than just getting shocked off of the hot wire while the connected neutrals are taking back most of the returning current, you can get hit harder off the whites as you can share a neutral between two circuits on different phases, and if there's no lights or anything taking up current as a load, you get it all, 220. 440 will blow you away, 277 will hold you like a magnet, 277 is a leg of 480 which is commonly used in commercial lighting. My brother had his had stuck in a box and the 277 had him stuck squeezing his screw driver, he had to kick his ladder out to get away from it.
Also if you have to hang a light with unidentified wires, use a continuity tester to the hot in the socket to keep your wires straight, you don't want the large threaded sleeve of a light bulb socket to be wired to the hot or if the bulb is out and the switch is in you can get shocked easily to ground, it's much harder to stick your finger all the way in and hit the hot tab (old illegal California 3 ways create this problem since switching reverses the hot and neutral wires instead of using dedicated travelers). Old cloth wiring can be deceitful and needs to be tested, sometimes the white wire cloth is dirty and looks black, people get hots and neutrals reversed sometimes because the white cloth wire is dirtier than the faded black one. If you suspect s ground neutral bonding anywhere other than the main panel itself, or a neutral ground reverse, you can use amp clamps around the metal gas and water lines to see if there's current (.2a is what they say can be lethal, and it can be hard to determine how much the system is actually taking since the water will absorb the current and act as a load. If there is an amperage reading with the clamps around water pipes and gas pipes it could be from a neutral ground reverse (found one where on a pull chain light with an outlet they had it miswired on the outlet plugin) or the current reading with the amp clamps around the pipes could be coming from a voltage leak, say from buried underground electric, feeding the house or bad underground wiring feeding say a yard light. Also the neutrals never are on the same bar as the ground wires in a sub panel, they must sit on a floating neutral bar, if using a regular panel tub as a sub panel you must remove the ground bonding screw from the neutral bar.so the metal panel and grounds are not touching the whites anywhere but back at the main panel.
Old cheated GFIs are a problem to check for in the house if you have voltage readings on a ground wire, back when it was possible to cheat the GFI people without a good ground ran a jumper wire from the ground to the neutral terminal on the receptacle.