Several years ago I was putting together a Pentium MMX200 system. I had the case lying on its side when I first powered up. The thing was dead, apart from the LEDs and fans.
Suddenly there was a loud bang and something whizzed past my ear. As I was reaching over to disconnect the power, to my shock and amazement, the PC beeped and then started to boot up!
Examination of the board revealed an electrolytic capacitor that had been soldered in the wrong way around. Presumably this caused the 5V power to be low, so the system wouldn't boot. Once the offending component had blown itself off the motherboard, full voltage was restored, and the system proceeded to boot!
I was very lucky not to get struck in the eye by the capacitor.
