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".
Nobody is on the streets selling psilocybin like they sell other drugs, it would be like people trying to pass off broccoli as marijuana.
I'm sorry but you sound like Nancy Reagan right now. You are trying to warn people about something that you yourself have barely experienced or knowledgeable about by taking some other tangent item and applying it here because it is "drugs".
They might have picked some "little brown mushrooms" in a field somewhere thinking they were psylocybin mushrooms but not gone to the effort to identify them properly, or just not known how to do it, or made a mistake. Or maybe they got it from someone who told them that's what they were, but themselves didn't know, etc.
Or they (or their supplier) might have gone the easy route and bought some mushrooms and sprinkled some "bath salts" on them.
As a consumer you just don't know -- especially if they're dried, crumbled mushrooms which could be just about anything or a mix of stuff.
It's just sad hearing people who are so adamantly determined to stick their head in the sand and trust whatever drug they're being peddled is what their dealer claims it is.
Nancy Reagan's famous line was "Just say no." I'm not saying "Just say no". I'm saying "Just say know."
Educate yourself. Test your stuff. Be smart. Don't just guzzle down whatever you're given without satisfying yourself that it actually is what your provider claims it is.
I heard more than one story, e.g. giving unsuspecting newbies tea under guise of weed. And that didn't even involve money, imagine how much you could earn by optimizing for finding shroom-curious laypeople. (If this is nothing to you, perhaps you are not poor.)
I'm not OP but I wouldn't want to take a gamble buying illicit substance from someone I don't know and can't vouch for.