Comment 90 for bug 311716

@Terrax: I don't think your problem is related to this bug. I've seen the behaviour you describe for "battery mode", and it corresponds to having the "Reduce backlight brightness" option checked in the "On Battery Power" tab in gnome-power-preferences, then dimming the display on AC power using the corresponding FN key combo (or otherwise), then unplugging the AC cable.

It would appear that the "reduced brightness" is computed with respect to the *original* brightness on AC power, not the *current* brightness; that is why the screen turns brighter. You may file a new bug report for this behaviour if you wish, although it is debatable whether this is an actual bug... It's almost certainly not a bug in the kernel.