Back to the subject: I transplanted the plant in question, and the old soil had a bunch of tiny white balls in it. It also had an occasional small patch of mold. I will post pictures to see if this is mycelium or what. Hopefully someone knows, because it didn't look white and stringy like SMOKEnCHOKE talks about. My wife took the camera to work, but hopefully tomorrow I can put up a couple pictures.

Upon transplanting I think I found a possible reason as too why I got mold and mushrooms. For drainage purposes, I had some clay pellets at the bottom of the container. These help hold some moisture. When I would feel the soil, I don't think I ever felt all the way down to the pellets. Even when the top 95% of the soil seemed completely dry for many days, there was still a bunch of moisture at the very bottom of the container. The roots were entangled in the pellets, which I believe never really had a chance to come close to drying out, which is of course necessary for healthy soil. I'm pretty sure this is what happened, and I hope this can act as a lesson for other people. I know it's a lesson for me.