Heineken magnum - was actually REALLY good

A friend opened a magnum of Heineken (under cork stopper) last night, said they got it from a friend 8 years ago or so, and it was quite a bit darker than regular Heiney, and actually tasted really good, though a bit flat. I wonder if the dark color is sun damage, or if it was a different brew of some sort - either way, it was way better than standard Heineken.