So this weirdest thing has happened to my Mac.





It only affects the left furthermost USB-C port. When the charger is connected to the Mac (through any of the remaining 3 ports), this one port will supply 20 volts on standby instead of the regular 5 volts, effectively frying any regular USB peripheral that's connected. When the charger is not connected, the port operates normally and supplies 5V.





The test setup is as follows: Apple USB-C to USB adapter plugged to left furthermost port, to which a regular USB cable is attached which exposes convenient measurement points for the multimeter.





Here's measurement taken of the voltage with charger disconnected:









and with charger connected (to any of the remaining 3 USB-C ports):



