I have used old cans of Mountmellick Stout with good results, obviously I couldn't tell if the beer was darker than it should have been!
In around '94 or so, our club was having a competition. The high scorer in the Munich Dunkel category was quite amused that he had won. When I asked him why, he told me that he had used 3 2.2#cans of very old Blue Ribbon malt extract! I don't recall where he had gotten the stuff, but it had turned very dark... the stuff should have looked like a pils, but nailed a beautiful dunkel shade! He had used a good, clean yeast, fermented it on the cool side and... well.. made a pretty darned good beer!