A while ago I was using my iPod video (5.5G, 30GB) with a self-made aux-in on my car stereo. Unfortunately my electronics knowledge was even more limited at that time and I did not realize the tape head was biased at ~12V so I connected the ground pin of the line-in directly to the common/ground of the tape head. This caused my iPod to reset each time I tried charging it with the jack phone connected. A few months later I switched to a 12V power hub which included a really crappy USB port as well. The moment I tried to charge the iPod while playing smoke came out of both the supply and iPod. In hindsight this was a stupid idea since the ground voltage of the charger was at 0V and the ground voltage of the ipod (headphone and usb) was sitting at 12V (it should have been 6V but the cassette player had a boost supply).
Electronics and programming interspersed at various levels of difficulty.