[atoav]

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.

[nicolaslem]

Another example of firmware update reusing something that is already there: some electric guitar effects from TC Electronics can be reprogrammed by bringing a phone close to the guitar pickups. The phone app encodes the data as sound that is picked up by the guitar pickups and sent down the chain to the effect.

[atoav]

Ahh good example I remember having done that myself before!