Amazing what biodiversity turns up on peat-free composts! I get all sorts of little mushrooms turning up - none of them seem to do any harm, even in seed pots and almost every pot (including the seed pots) has an earthworm in it. I don't know how they get there, and contrary to what most of the literature says, they don't seem to do any harm at all. It has been suggested that the presence of a healthy soil ecology (unlike in peat-based composts) prevents certain pathological organisms, such as Pythium (damping off) or vine-weevils, from getting out of hand, and certainly I tend not to see either (touch wood).