I saw that once in a computer lab that had all-metal desks. There was on desk in particular that if you grabbed it and a nearby desk, you would get a strong although not painful continuous electrical shock. The sensation was very similar to those trick shock pens that you find in novelty stores.

I would say that that should not be normal. Anything that does that is defective. If it is a normal occurrence with Apple power adapters, I would say that those power adapters are categorically defective.