Actually I've never found a cache that is a cold wet shopping bag. All sorts of other containers, to be sure.
I think he means an ammo box or tupperware wrapped in a cold wet shopping bag, placed by the cache owner with the horribly mistaken assumption it would protect the container, when actually plastic bags attract, rather then repel moisture, and are a breeding ground for slimy, creepy crawly insects. It's pretty much a regional thing in some areas, a "monkey see, monkey do" sort of thing.
Speaking of which, I see many regional "oh no's" in this thread.
Sign me up for Oh No, not another micro in a pine tree which I believe DocDitto said. Can't see them to save my life. Even if it's a decon container, which is not even a micro.
I am not anticipating the day that I reach for the black garbage hidden under a tree in the middle of woods, and find out that there is no cache inside, yet something else much less desired.