I managed to install a PC floppy drive power connector upside down! I closed the case, powered up and in seconds thick white smoke came belching out of the PSU fan outlet! I thought it was the PSU on fire or something, but it was "only" the insulation on the floppy power cable melting and burning as one of the wires glowed red hot with the short circuit current.
I've also killed the video output on an A1200 by plugging in the monitor lead to the 23-pin connector while the machine was powered up. A quick spark and no more video from that motherboard.
Not sure which was the worse disaster!