When I built my first Sandybridge PC which was a massive upgrade over my previous Athlon 6000+ PC, I plugged the front USB cable into the Firewire header because I wasn't paying attention and the colour & pinout were the same. I turned it on and the motherboard caught fire near the header.
I still used that motherboard for 2 years, I de-activated Firewire in the BIOS, cleaned the burn mark with something or other, and put a blanking cap over the header. Sold it for £30 still.
Another thing I did once was take the fans off of my 240MM radiator which was in the top of my case, which of course, meant taking out all the screws which were holding the radiator in. I then continued what I was doing, testing the temperatures with a passive radiator, and then I felt my heart sink through the floor as I suddenly realised that the only thing holding it in was the warm rubber washers sticking it to the roof of the case. I managed to stick a screw or two in before it fell off though!