I had to dump my batch of pumpkin ale that I brewed up last month. It had been sitting in the secondary fermenter for about two weeks, and I was going to bottle it today.
When I got the carboy of beer out of the pantry, it was all too obvious that it had become infected: large growths of pond-scummy tan chunky-looking… something. (See pictures below.) Definitely not what you want to see floating in your beer. Removing the airlock and taking a whiff confirmed the diagnosis: infection of some kind. It smelled kind of metallic, and simply "off." So, I poured it out.
Sucks to have to do that to five gallons of beer, but at least I hadn’t bottled it yet. (I had sanitized my bottled today, though.)
So what went wrong? It’s tough to say for sure. First off, the pumpkin was old—it had been in the freezer since last November or so. And when it thawed, it sat in the fridge for an extra day or so. Another possibility arises from when I was racking the beer from the primary to the secondary fermenter—I added the spices and vanilla at that time, to see how they would come across in the finished product. (My previous pumpkin beers had the spices added at the boil or at bottling time.) So something could have been introduced then.
Or perhaps it was just a random infection; sometimes they happen—the carboy isn’t completely sanitized somehow, or something went wrong early on and didn’t show up til the end. I’ve only had one other batch go bad like that and have to be poured out, and that infection didn’t show up until a few weeks after bottling; it was a viscous infection like this one, only growing in the bottom half of the bottles (rather than floating at the top).
At any rate, sucks to lose a beer. Check out the pictures below (and maybe other brewers reading this will have some insight into just what type of infection this was from seeing it):