When I had almost finished my keg of "quaffable porter", I decided to bottle the remainder with my new beergun. These were only four bottles, it appeared. When I tried one yesterday, a couple of weeks later, the porter from one bottle had an unpleasant, slightly sour flavor. Today I checked another one, same flavor. Completely unlike the kegged beer. So I wonder what has caused that. I'm pretty anal about the bottles themselves. Also, there was no additional carbonation, to the contrary, I had some CO2 loss due to bottling with the beergun. The beer did not really taste infected either, just different and a bit more sour than the kegged version. But can it be anything else than an infection? Also, I have a keg with pressurized star san that I use to clean the beergun, and I'm pretty sure the beergun line was completely empty when I started filling.
I guess you are going to say the beer picked up something, but what, and where?