"I've never heard of someone buying "counterfeit" magic mushrooms."Just because you haven't heard of it doesn't mean it doesn't happen.
First, mushrooms can be misidentified, due to carelessness or ignorance. A misidentified mushroom can be poisonous. So that's my primary concern.
Second, it's easy to just add whatever chemical you want to non-psychedelic mushrooms, so it's quite possible that you'll get some non-psychedelic mushrooms with some mystery drug added to them.
Third, if someone on the street (or even a friend) offers you something they claim is psilocybin, there's no way of knowing if it really is psilocybin unless you test it.
Figuring out the substance's purity/dosage is even more difficult, as labs in the US that do commercial drug testing aren't legally allowed to report purity, and reagent testing isn't going to tell you.
The amount of psilocybin/psilocyin in mushrooms also varies greatly, so going by gram of mushroom weight (dry or fresh?) isn't very reliable.. especially if you don't know exactly which species each of the mushrooms are and someone's just selling you some mystery "magic mushrooms".