I think they meant it is not uncommon for someone to mistakenly rewire a switch so that the neutral line is switched.
In high school I worked in a hardware store (where I first learned how a 3-way light worked) and you would be surprised at how often someone would want to return a light switch as faulty after they incorrectly wired it. The symptoms were always the same. They turn the breaker back on after replacing the switch and either the breaker trips immediately, or the light is on and when they flip the switch the breaker trips. Inside the switch box they would connect white to white and black to black (because that is always the way to hook-up wires, right?) and then hook white to one side of the switch and black to the other. God only knows how many switched the white wire and were happy they "fixed" the switch -- until they try to unscrew a broken light bulb.
In high school I worked in a hardware store (where I first learned how a 3-way light worked) and you would be surprised at how often someone would want to return a light switch as faulty after they incorrectly wired it. The symptoms were always the same. They turn the breaker back on after replacing the switch and either the breaker trips immediately, or the light is on and when they flip the switch the breaker trips. Inside the switch box they would connect white to white and black to black (because that is always the way to hook-up wires, right?) and then hook white to one side of the switch and black to the other. God only knows how many switched the white wire and were happy they "fixed" the switch -- until they try to unscrew a broken light bulb.
Here are many more examples: http://thereifixedit.failblog.org/