Sparks when plugging in USB
Is this right? 


Yes, this can happen.  

Both devices, the iMac and the 12V power supply will will have a Y-class capacitor.
The iMac will have a mains filter. With CY 1nF to Earth.


The power supply (assuming cheap wall-wart) will have a Y-capacitor.


Causing a small current to flow.
Usually this isn't a problem when you have grounded sockets. But if you do not have grounded sockets, the current will flow between the power supplies, though your board.

It sparks because the circuit changes from infinite resistance to a short.
This can cause damage to sensitive components if these are the first to make contact, or cause chips to glitch or reset if the impedance of ground is high or chips are improperly bypassed. Which is why the shield on USB, VGA, HDMI connect first, then the power and the signals last. Take a look inside the plug to see the pins with different length.

Sometimes you can mitigate the effects of these capacitors by reversing one of the plugs, and making sure you disconnect live when using a switch.

You do not want to remove these capacitors, because instead of a clean 50/60 Hz leakage current, you will then be looking at kHz or even MHz leakage and common mode noise.