When I load a do-nothing sketch onto my Arduino UNO R3, the pin 13 LED lights up. My understanding is that by default all pins are supposed to be in a high-impedance state. Have I burnt out that pin?
I think we had this one recently. But the search feature isn't that good.
I think pin 13 may be HIGH but INPUT. The effect is that it will appear lit up. In other words, it is high-impedance but with a weak pull-up. An artifact of how the bootloader leaves it.
The pull up may be enabled for some reason.
Try explicitly setting the pin low.