This is a bit long, but here goes.
I was tired one day, and hooking up my old 98SE PC to play a nice old dating sim. I needed the floppy drive, so I shut her down and plugged in the power cable without moving to look at how it was plugged in. I just jammed it into the slot and booted it up, spinning back to my normal desktop. After a few moments, I looked at the CRT monitor. "HARDWARE FAILURE!" under all the other BIOS crap. I shrugged and looked at the box. It was smoking and smelt like bad burnt chrome. I hurredly scrambled for the power cable and pulled it out. Looking inside, I'd plugged the pins in wrong for the floppy. It was a 4 pin, and only three were connected. Needless to say, most of the other cables were also melted together from the heat, and if I had of been a few moments later, I may have risked a bit more than a power source and a floppy drive.