That's the one, with the finger over the sensor. Happens by mistake, happens in car, etc. Popping the kbd ensures it doesn't blink on and off as I drive.

Also, the sensor isn't idiotproof. E.g., gets crazy from direct sunlight. Or rotating the phone with steering wheel. Or hair, if one has long hair and hols phone not at ear but not on table either, which is common in high-noise environment and if you are a girlfriend of a N900 owner

Also, my said gf also has small ears, and refuses to smoosh the phone against the face until red (which I do because I am willing to suffer for hi-tech stuff), so it goes speaker-phone-speaker as the hair moves by or she moves.

I vote for kbd override.