All of the above answers are correct. You CAN get zapped on a circuit even with breakers and switches turned off. I've been working in electronics and electrical stuff since 1964, and I *usually* change out light switches and wall sockets at home with the juice turned on - BUT I'm always fully aware of which wires are which, and I don't allow myself to get between two poles of a circuit or between the circuit and ground. I act like a bird sitting on a high voltage line, which is pretty safe as long as the bird doesn't touch anything else at the time.
When I've worked in the capacity of an industrial electrician, everything is done by the book - padlocks and lockout tags on the circuit breaker or switch, probe the circuit with a voltage detector AND a voltmeter, before touching anything that might be live, don't work alone, etc, etc.