Ive heard people say one bad one in a bunch of good shrooms can kill you, but do they mean bad ones grow with good ones or what?
No, mushrooms don't often mix themselves together because the ground beneath them is colonized by the mycilum making it impossiable for other things to grow because there is no room for them. It is possiable though for a novice picker to fuck up and add a bad one to the harvest... Personally when buying shrooms from someone you do not trust, I would only buy cultivated mushrooms.