Floating supply and USB ground
From my knowledge:

Floating power supplies can be measured with a multimeter in AC mode in regards to ground, in my case, axp power wall plugin. It will measure 130 Vac, it will give out 12 V dc, but remember this is not entirely true. Instead, you will see a sinusoidal signal on both the + and - output. This is perfectly fine, as you put the minus on the PCB ground and the plus on your Vcc, so the voltage in between those two will always measure 12V dc. Now what happens if an external grounded device, like usually USB comes into play.

You will connect a real 0V GND to the 130Vac gnd of your PCB, now if you hit the zero crossing, of course nothing is going to happen. If you hit the 130V point, you will actually shorten 130V to gnd, since there is not a lot of power behind those 130V. Most of the time it simply turn to 0V without nothing, but sometimes, USB will be reconnected or µCs will restart or or or...

So I am also curious on how other engineers found a solution to this problem, as I am constantly struggling with this.