LG appliances at least used to use acoustic signaling for diagnostics: hold a phone up and the washer makes some modem-esque (I think it’s 4-tone / 4-FSK) noises and the app or technician can diagnose issues. It was originally engineered to even work over voice codecs, so a customer without a smartphone could relay the diagnostic session to a technician.

That's pretty cool. I found a write-up[1] on it but unfortunately didn't come across any examples of the communication.

[1] https://github.com/kabelincho/LG-Smart-Diagnostics-modem

That's some advanced gatekeeping right there. Where other appliances might have a blink code or several digit error display (Miele) to look up in a manual, the phone method tires you to the manufacturer.

The absolutely "leetest" thing I have ever seen was a device where the firmware update was to be done by:

1. Hold a button while booting (pretty normal)

2. This reconfigures the circuit path of one of the LEDs so it is reverse-biased to VCC via its resistor and switches one of the microcontroller GPIOs to ADC input

3. You go to a website that plays a strobe pattern (encoding the firmware)

4. You hold that website in front of the LED till other LEDs blink, signifying a successful update

They could have done this using a photodiode, but no, they had to abuse an LED. Not many people are aware that LEDs can in a certain configuration be used to measure light.

>so a customer without a smartphone could relay the diagnostic session to a technician

Do you mean by mimicking the noises themselves?