I was excited to try cooking with dried mushrooms until I saw some comments from people who found (many) dried worms mixed in with their dried mushrooms. How common is this?
It must be pretty widespread (at least in morels) as evinced from this pretty extensive list of references to worms in fresh (and dried) morels. From what I've read, the presence of worms seems to indicate that the mushrooms are of lower quality and might have been harvested too late.
Dave Fischer's North American Mushroom Basics - Some fungi have evolved to take advantage of multiple food sources. For example, the Oyster Mushrooms you can buy fresh at many grocery stores break down and digest cellulose, but they have also developed mechanisms for literally trapping and then eating tiny little "worms" called nematodes; this gives them access to extra nitrogen...".
Mycological Society of San Francisco - Avoid morels whose caps are soft or mushy, or become granular when rubbed: they are too old and wormy. Morels occasionally contain insect larvae that drop out during the drying process. The mushroom-lovers we know have disregarded this aspect of morel enjoyment. After all, they are very small worms. Because of the irregular nature of its surface a morel cannot be rubbed or brushed. You may find this worrisome, wondering about what kind of things lurk in the dark pits ready to jump into your b'chamel sauce...
Sandy’s April Product Report - When purchasing morels, check carefully for worms and excess sand. The worms should be easy to spot as they are white, in contrast to the dark mushroom. To clean, gently brush off any sand or dirt with a soft pastry brush.