Inserting the 4-pin molex power upside down, frying a 5.25 external USB enclosure, DVD-RW survived amazingly.
Removing cooler of my ancient "Morgan" Duron 1.2Ghz while the pc is on thinking it should shut down to protect the chip but it went kaput after a few seconds. Still had no idea why I did that in the first place think I was just bored with the CPU.
EDIT - oh and plugging in 4-pin molex power to the HDD while the PC is on, there was a nice sparkle then the PC shut down. It worked again though after unplugging the main several times.