> MacBook

Do you mean the American Electric "Suicide" Computer?

Yes I've been zapped by it numerous times.

Zapped by shower heads? Don't know anyone.

Almost died from a gas heated shower because of a leak? I know one person.

Yes, this is anecdata.

Y capacitors in power supplies cause a small leakage current. If you are using an unpolarised plug you may get a lower voltage by rotating its orientation.

http://www.aplomb.nl/SMPS_leakage/Doc_ie.html

Yes that happens when the power supply isn't grounded. Use the extension cable instead of the duckhead as it connects the ground pin.

do you have a hygrometer available? below 50% relative humidity, I get zapped every time I touch my mid-2012 mac book air. I've taken to drying clothes in my living room when the relative humidity goes under 50%.

Apple kit is awful for that, same applies with phones too.

Yep I do. Metal casing gives me the tingling.

me, that's me