Due to EMI reasons, for switching converters, the hot and neutral terminals of mains input is generally capacitively connected to the Earth prong via high frequ

Due to EMI reasons, for switching converters, the hot and neutral terminals of mains input is generally capacitively connected to the Earth prong via high frequency capacitors. That means if the earth prong isn't properly connected to actual earth, the earth prong now has your mains voltage divided by two on it. If the earth connection happens to be connected to the chassis, there's your shock. It probably won't kill you unless you have a weak heart or if you're really unlucky because the EMI suppression caps are small enough to not supply lethal current, but can be really annoying.