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.)
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.
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.