I have a rMBP and have brightness set to the highest setting when plugged in. The box to slightly dim the display when on battery power IS checked. The box "Automatically adjust brightness" is NOT checked. For some reason, the brightness setting is not holding properly. For example, when I unplug the laptop, brightness does decrease as it should, however, that reduced brightness level is not always retained and brightness sometimes reduces further even though I haven't done anything to adjust brightness. This seems to happen more when on battery power, i.e., when it is plugged in, it seems to hold the maximum brightness setting better. I've now unchecked the box to slightly dim the display when on battery power to see if that changes things. I've already done a clean install of Mountain Lion, but I still assume a program I'm running or something inherent in ML is causing this as I doubt this is a hardware issue. Is anyone else experiencing this or does anyone have any thoughts?