I've had something happen to a couple batches and can't figure out what's going on. What happens is a major flavor change after the first two weeks of primary, but for the worse. Normally my beers keep getting better by the day wether I bottle or keg and seem to mature, but a couple batches have basically taken a bad turn and come out really similar even though they were completely different beers (Imperial IPA(bottled) and Helles Bock(keg)).
The helles bock was clear and bright after a two week primary and two weeks of lagering and then I moved it to a serving keg (with a jumper hose to not get any oxygen in) to get it off the settled yeast, and lager a bit longer. I sampled a couple glasses right after the move it was excellent, clear, tasted clean and crisp. Now after two weeks sitting in the fridge it is horrible! Also it seems to have gotten cloudy too.
One of my only guesses would be oxygenation, but the only comments I find claim oxygenated beer tastes like "wet cardboard"... don't really know what that would be, but doesn't seem like that to me.