Why did my Arduino Uno give me a shock?
The power supply of your PC has a mains filter. That one has some capacitors between both mains poles and the grounding. The mains ground is also tied to the case of your computer through capacitors and that one to the ground pin of USB.
All the &quot;grounded&quot; parts are at half of the mains voltage unless the grounding of the building you are working in is superb. I have not yet encountered such a building.
The current is however sure less than one milliampere. LEDs light up from a few microamperes already, just not very brightly.