When adding a daughter board to a PC we used to leave the mains cable connected...
I did that, once, when adding memory. Destroyed the memory and the motherboard. In the manufacturer's defense, there was a lit LED on the motherboard, which I ignored, because I was tired, inattentive, or both.
Lesson learned.
Mains cable is now always unplugged before opening the case.
Biting the hand that feeds IT
