This is incorrect. As the battery charges, the button will indicate the color that the current charge is. Yellow at about 3.6 volts turning to green at 3.7 volts and higher. As soon as the battery is fully charged, the unit will stop charging and the button light will extinquish.
You might have had a battery indicate 4.2 V (which is full charge) but it is not yet fully charged.
Leave your battery charging overnight and you'll wake up with the light fire button light off.
The green light on mine doesn't go off even after charging for over 3 hours!! yet it shows 4.2 in about 2 hours. Either mine is faulty or the minimal charge between 4.2 volts and full is just plain messed up. I haven't charged it overnight, but it shouldn't take over an hour to reach full charge from 4.2 volts. And besides, an overnight charge on a 850 mah battery is ridiculous in itself.