In this case my chip was repeatedly exposed to daylight (direct sunlight) before I realized the sticker had fallen off. Typically, the chip is exposed to UV intentionally to erase it to prepare it for new programming. I certainly wasn't trying to do that. In most cases, I don't think it's [being exposed to regular light] anything to worry about, but it wouldn't hurt to make sure the sticker doesn't fall off. I should also mention that the unit didn't stop working all at once, so it could easily be one of the other two issues Steve mentioned.