USB port and bench power supply ground difference
You have a bench supply and USB supply with AC mains feed.
 These may have two wire or 3 wire (phase + neutral + ground) leads.
 The outlets used may or may not have grounds.
 If both outlets and leads do not provide grounding so that both power supplies have third wire grounded input grounds then the following is likely (based on what you describe and my experiences):

- You are very probably getting coupling from mains via a Y capacitor in each lead of mains to input ground.


If you measure the 0.4 VAC with no load and a decent AC meter you will probably see about Vmains/2 or 55 VAC or so in USA.

- This capacitor coupled mains voltage can destroy things.
Y caps may be 0.001 or 0.01 uF. 


"More data needed" including certainly whether PSU ground is connected to mains ground, what a "Bus Pirate" is (Gargoyle will for sure know but I/we ought not need to ask it) etc. 

BUT some supplies have isolated in and out DC wise and  Y capacitors from phase &amp; neutral connected to input ground. If you use a 2 wire power feed and connect in-gnd to out-gnd you get mains/2 capacitively coupled. If you are getting bitten you have >>> 0.4 VAC and probably half mains via Y caps.