Did the beer taste different too? This just happened to me. It was a spotted cow clone from Northern Brewer. No signs of ill fermentation, and when I transferred it, it looked and tasted spot on. I wonder if the used keg has some residue from a soda I can't seem to clean. I've used PBW, following the directions, and have even been flushing with star-san after rinsing out the PBW. Thoughts?