I read a great how-to once where someone described a device which blinks in the dark - and only in the dark - that uses the LED itself as light sensor.
At work we replace an obsolete diode in part of a circuit in front of a high gain amplifier. The new equivalent diode came in a glass package unlike the old one. It caused a bit of head scratching to figure out why the circuit wasn't behaving as it should and also appeared to be acting as a human proximity detector.