This reminds me of a time in college. Everyone's project was to build a regulated power supply and used polarized capacitors to filter the ripple. The first guy to plug his power supply in scared the class as his capacitor had been soldered in backwards and exploded, launching it into the ceiling. Everyone else was apprehensive to plug theirs in after that event.
The guy beside me was about to test his out. So I cocked the ruler against the test bench, and just as he plugged it in, I let the ruler snap against the bench. There was no explosion but some expletives were involved. Good times!