A related thing to watch out for:
If you have a metal-cased computer (or other equipment), and the PE (3rd prong) wire is not connected properly, then the two capacitors in the line filter will pull the metal case to half of the mains voltage, in series with a reasonable but not trivial capacitance. The capacitor should be chosen such that 60 V or 110 V (depending on the local mains) should not electrocute a healthy person, but you can easily receive a jolt that could knock you off a ladder or other slapstick cause of injury, especially in 220 V lands. At work, we called this the "Dan effect", as in "it's not easy to shock Dan, but..." after he encountered this phenomenon in the field, where the locals had not bothered to ground the generator properly.