What a bummer, Jinx :-(

Other replies have given you great crowbar circuits but, I do have one idea
about how it may have failed.

IIRC, this type of failure can be caused if the input is shorted and you
have a lot of capacitance (>5uF) on the output of the 7805.

An input short failure can be prevented by adding a diode from the input to
the output of the 7805. IIRC, the National Semi LM340/LM7805 data sheet
mentions this type of failure and the diode solution.

Paul

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