I've found feathers, bits of cotton, string, among other things in bags of weed. It's really not too big of a surprise when you consider that the bulk of ganja is grown outdoors. I've also heard of dealers putting "markers" in the sacks they sell. Though that seems to be a lot more common with the powders and whatnot that are a lot harder to tell apart than marijuana. There was a large batch of some commercial buds going around here last summer that had one very distinctive brown leaf in every ounce.