People have died trying to pick psychedelic mushrooms in the wild and getting it wrong. Then again, that danger exists with regular food mushrooms in the wild as well. You just really need to know what you're doing.

From a dealer, you're very unlikely to get toadstools, for the reasons you described, but non-psychedelic mushrooms (even from the grocery store) are frequently dosed with cheap and often legal, but dangerous "research chemicals" and sold as psilocybin mushrooms. This was done with LSD back when it was cheap and readily available as well.