I was cleaning out my fridge and had a couple bottles of yeast slurry that had been sitting in there for well over a year. When I dumped one there was a chunk in it. It looked strange so I rinsed it off. Here's what it was: Not sure how it got in there. Since I would surely have noticed it when cleaning/sanitizing the bottle, I'm guessing it got into the fermenter, died, and ended up getting trapped in the yeast cake. Then, when I collected the slurry, I must not have noticed it.