Reverse-polarity protection
Circuitry that makes connecting power backwards a non-event instead of a funeral. The modern version is a P-channel MOSFET that only forms its channel when polarity is correct — near-zero loss, no fuse to replace.
In practice
A series diode also works but wastes ~0.5–0.7 V forever — painful on battery rails. The P-FET trick costs millivolts. Anything a user plugs in (battery packs, barrel jacks, screw terminals) deserves it; one assembly mistake pays for the part a thousand times over.