Electric shock from cover of preamplifier
If it's a wall-wart USB power supply, it has a floating secondary. It will develop an offset with respect to ground which, while annoying, isn't harmful. It can lead to hum noise.

The loud 'poof' you felt the second time was probably static discharge from your own body.

Try grounding your preamp or power it from your PC and make sure it's grounded. This will address both issues.