Body diode
The parasitic diode inside every MOSFET, from source to drain (in an N-channel), formed by the transistor's own structure. It conducts whether you want it to or not — many circuits quietly depend on it; others are quietly broken by it.
In practice
Reverse-polarity protection exploits it: at first contact, current flows through the body diode, then the channel turns on and shorts it out for near-zero loss. In bridges it freewheels during dead time. Its slowness (reverse recovery) is also why fast converters pair FETs with real diodes.