I opened up my 1996 bottle of Samichlaus the other night, and found that it had gone bad. Not horribly bad, mind you, but enough to be undrinkable. I tried, though, but after a few sips I just had to pour it out.
I blame my storage—obviously I didn’t keep it stored well enough to last. Too much light and/or heat, probably. And it has moved with us over the years, at least three different houses, so who knows for sure.
When I poured it, it was a viscous brown sludgy-looking liquid; no head at all, and you could see particulate matter floating around in it. It had a strong oily, woody aroma to it (my wife described it as "like echinacea") and a corresponding taste. Woody, a tiny bit metallic. Just enough to know that it wasn’t good any longer.
Properly cellared, a high-alcohol beer can be stored for years without detriment. Come to think of it, a cellaring how-to would make a good article (or series of articles); I can’t say as I’ve seen many writings on the subject. Something for 2007!