Reverse polarity protection in PCB design
I'm trying a fuse followed by a reverse-biased diode.  With reversed polarity, the diode conducts and takes out the fuse.  In fact, I'm using a unipolar transorb (e.g. SMBJ24C), which will also conduct on over-voltage with the same result.
In one application, I've even followed the above with a forward-biased diode - to prevent the transorb from having to discharge large input filter capacitors.