Sparks = damage.

Today I discovered why short-circuiting the input side of the robust LM7812 voltage regulator will destroy it.

I've had train wrecks of series of regulators dying from a single short-circuit on the input side of things. This was most unexpected. The cause is the capacitor on the output side trying to push power backward through the regulator. It can be prevented by connecting a diode across the input and output.