[derefr]

I long ago stopped using the two-prong adapters; they're just a bad idea when you plug in at places that don't have particularly clean power. Where I used to live, my unibody Macbook would literally give me a little static shock whenever I touched it as long as it was plugged in with the adapter (or possibly it was a continuous current; running your palm along the metal felt a bit like rubbing a balloon.)

Switching to the long 3-prong "cable" connector solved that, and I've never dared go back.

[digi_owl]

And this perhaps a good reason why the whole metal case on mobile phones is a bad idea.

What you are describing is grounding problems of one for or another.

[derefr]

(I'm not an EE, so if anyone knows better?)

Oddly, never had the same problem with a phone USB charger. Which is the opposite of what you'd expect: the USB chargers seem to be entirely solid-state, using flyback transformers[1], while the larger power adapters have room for a real wire-wrapped ring transformer[2], which should be providing better electrical isolation. (I would never expect to be shocked by dirty power coming through an induction ring.)

[1] http://www.righto.com/2012/05/apple-iphone-charger-teardown-...

[2] http://www.righto.com/2015/11/macbook-charger-teardown-surpr...

But if it is the device itself that was faulty, rather than the input power, it's a different story; in that case you want at least some valid ground path, and complete isolation is bad (because then the device will ground through you, obviously.)

Maybe it's just that the power adapter block has a separate ground path and expects to ground it into a dedicated ground pin (and, I guess, leaves the ground pin disconnected when a two-prong adapter is connected? That seems wrong-headed), while the USB chargers, made to assume two-prong usage, mix ground with positive.