My most recent endeavor is a Belgian dubbel, a fairly strong ale that originated among European Trappist monasteries. I brewed the dubbel last Wednesday (5/22/13) afternoon, and ran it into a primary fermentation tank before pitching the yeast.
I noticed at the time that the yeast starter packet had not expanded as it was supposed to have done, but pitched it into the fermenter anyway. I later discovered that the yeast I was using (1214 Belgian Abbey Wyeast) is notoriously slow to start, and that I probably should have gotten the starter packet working the night before I brewed. Because the yeast was not fully activated when I pitched it, it took a lot more time for the yeast to actually take off, giving more time for infection to set in.
I sealed the fermenter and plugged it with an airlock, but after 24 hours there was still little to no outward sign of fermentation—no bubbles in the airlock, no foam on top of the wort, etc. That evening, Alyssa and I went out of town to visit her parents for the weekend. Four days later, we returned to find a little bit of a collapsed foam cap on top of the wort, accompanied by a bad smell (like sewage), in addition to this oniony looking stuff floating on top of the beer:
Fearing that I had my first infected brew on my hands, I went to Bacchus & Barleycorn, a local brewing supply store in Kansas City, and showed them the above photo. They said that it wasn't actually mold (although the were uncertain what it actually could be), and that the best option was just to ride it out and see what happens. They explained that the smell could just be due to the slower fermentation speed.
Infected beer isn't bad for you (the ph level prevents pathogens from growing in the wort), so I may just go ahead and bottle it and hope that it doesn't affect the flavor too much.