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The breaker and/or wall switches only turn off the hot line. The neutral and ground wires are not switched. In almost all cases the neutral and ground wires really are at ground potential, but very rarely they're not at ground potential due to a poor or failed ground connection at the breaker box or on the line drop coming in from the pole. The neutral and ground wires have about a 99.999% probability of being properly grounded, but one should be aware of the possibility they aren't - and take appropriate precautions against getting between ground (copper water pipes, etc) and ANY part of the circuit being worked on.

If the work is done wearing rubber soled shoes, on a wooden ladder, on a carpeted floor, etc, etc, there's very little way to establish a path to ground through your body. You should always be as insulated from ground as possible, and have the breaker and switch for that circuit turned off.

Here's a scenario: You kill the breaker on the circuit you're working on. While you have the box open you find several neutral and/or ground wires connected together with a wire nut. You have to separate those neutral and/or ground wires to make a new connection. Those multiple wires in that wire nut might be the result of two or more different hot breaker circuits sharing the same neutral and/or ground wires in the return path back to the breaker box. When you separate those grouped neutral and/or ground wires, some of them are still trying to be the return path for energized circuits on another breaker that's still turned on. This "shared" neutral and/or ground return path is NOT supposed to exist in wiring that's done properly and according to electric code, but it really does occur far more often than you would think. When you break apart multiple neutral and/or ground wires that are all supposed to be a ground potential, you may actually be "ungrounding" the return path for another energized circuit on another breaker. The "ungrounded" return path thus created will go back to ground by any path you provide it - including your body.

If this doesn't make sense, let me know and I'll try to explain it more clearly.