My Laptop Bit Me – The Shocking Truth (2012)

Switch-mode supplies create a lot of spurious interference frequencies, and to prevent these from passing back into the mains supply (and cause havoc to other devices) ‘decoupling’ capacitors form a crude but effective low pass filter which shunts these unwanted noise-frequencies to the common ground. When the device’s common ground is
not connected to an earth, the common ground (which often includes the case, if it is metal) is energised to half the mains AC voltage, but at a very very high impedance. If you know your power supply has an earth pin and should be ground but you still get tingles and zaps, you have some serious mains wiring issue and should get a qualified electrician in to investigate as soon as possible.

Source: enclydion.blogspot.com