I experience this spam in .xsession-errors, repeatedly

(gnome-settings-daemon:1762): power-plugin-WARNING **: failed to restore backlight to 100: out of brightness range: 100, has to be 15 -> 0

briefly looking at the code, we find

        manager->priv->kbd_brightness_old = -1;
        manager->priv->pre_dim_brightness = 100;
 
and 
        gint                     pre_dim_brightness; /* level, not percentage */


100 looks more like a percentage than a level.