if the batteries are at half charge or drained and the light is direct drive, nothing will be damaged, just your lamp might not turn on until the battery is juiced enough to provide enough current to light the lamp
if the light contains a driver circuit such as a soft starter, depleted cells may have a nominal voltage at open circuit, but once the driver starts drawing current the voltage will sag dramatically...once it goes below the minimum Vin of your driver the light will not function, but there wont be any damage.
Overcharged batteries wont either :) , not on the Mag85, just your cells may reduce in capacity.
I find it incredibly hard to destroy the mag85...I use it as a utility light, other than the lamp and the window...everything else is drop-kick resistant
may I ask what charger you are charging your batteries with?